expense of enjoying his life, is the main theme of the story.

This story was written in 1956. This was during the height of the post-WWII boom where the American economy was going strong and men (very few women held good jobs) were striving hard to get ahead. This was a time when men did not spend much time with their families, choosing instead to work hard so they could advance their careers and thereby achieve more material security.

In this story, Tom is a real go-getter. He wants to work hard so he can be known as “the Boy Wizard of Wholesale Groceries.” Because of this, he decides to stay home and work instead of go to the movie he wanted to see with his wife whom he loves. He is giving up that sort of pleasure in order to get ahead. The story centers on what happens to Tom when he stays home and how this affects the way he thinks about what is important in life. The answer to your question, then, is that Tom stays home so he can get more work done and thereby advance his career and his family's material prosperity/ security.

The whole story revolves around Tom's decision to climb out on the ledge to retrieve the document he had been working on. The author therefore takes great pains to make his action plausible. He devotes many paragraphs to explaining what is going on inside Tom's mind while he is still inside his apartment looking out the window, trying to decide whether to abandon the paper or climb out and get it. The fact that his wife had gone off to the movies and left him all alone is very important, because it is pretty obvious that she never would have let him climb out the window onto that ledge. Women have better sense than men.

The first paragraph in which he agonizes over what to do begins as follows:

It was hard for him to understand that he actually had to abandon it--it was ridiculous--and he began to curse.

The next paragraph begins as follows:

For many seconds he believed he was going to abandon the yellow sheet, that there was nothing else to do.

But in the next paragraph:

But just the same, and he couldn't escape the thought, this and other independent projects, some already done and others planned for the future, would gradually mark him out from the score of other young men in his company.

And in the next paragraph:

By a kind of instinct, he instantly began making his intention acceptable to himself by laughing at it. The mental picture of himself sliding along the ledge outside was absurd--it was actually comical--and he smiled.

And in the next paragraph he begins to talk himself into actually doing what he has imagined himself doing. The thought that convinces him to climb out the window eleven floors above the street is this:

It occurred to him that if this ledge and wall were only a yard above the ground--as he knelt at the window staring out, this thought was the final confirmation of his intention--he could move along the ledge indefinitely.

In other words, it was only the great height that presented a problem. Walking along the ledge would be easy as long as he didn't lose his nerve. And he believes he won't lose his nerve as long as he just doesn't look down. The author devotes five full paragraphs to having his character rationalize doing something that seems nearly suicidal once he is actually out on the ledge.

It isn't until he is outside hugging the wall with "his shoe soles shuffling and scraping along the rough stone" that he realizes that "the ledge was not quite as wide as he'd expected." The reader has identified with Tom and is out there on the ledge with him in imagination. The reader can feel the insecurity of Tom's position because the ledge is not even as wide as the length of his shoe. His heels are actually touching nothing but empty space. But after a few more steps both Tom and the reader have passed the point of no return. It would be just as dangerous to go back as to forward.

Then the situation gets even worse. He can't pick up the paper without looking down at it, and he can't look down at it without seeing a large part of Manhattan eleven floors below.

And a violent instantaneous explosion of absolute terror roared through him.

The author does an excellent job of making it plausible that his ambitious young character would climb out on the ledge of a high-rise building and risk his life for a piece of paper.

It may seem crazy for a man to climb out of his window onto a narrow ledge eleven floors above the street in order to retrieve a single sheet of paper. This was a problem that the author Jack Finney had to wrestle with in order to get his reader involved with his story. Finney takes great pains to explain how valuable that sheet of paper is to his hero Tom Benecke. He was all set to write his report. That was why he was staying home rather than going to the movie with his wife. The story opens with a sentence which shows that he has reached his moment of truth. 

At the little living-room desk Tom Benecke rolled two sheets of flimsy and a heavier top sheet, carbon paper sandwiched between them, into his portable.

He is already thinking about what he is going to put into that interoffice memo. But the facts and figures are not in his head. They are all on the sheet of paper which, with the notorious perversity of inanimate objects, chooses to take flight and escape out the window.

It was hard for him to understand that he actually had to abandon it--it was ridiculous--and he began to curse. Of all the papers on his desk, why did it have to be this one in particular! On four long Saturday afternoons he had stood in supermarkets counting the people who passed certain displays, and the results were scribbled on that yellow sheet. From stacks of trade publications, gone over page by page in snatched half-hours at work and during evenings at home, he had copied facts, quotations, and figures onto that sheet. And he had carried it with him to the Public Library on Fifth Avenue, where he'd spent a dozen lunch hours and early evenings adding more. All were needed to support and lend authority to his idea for a new grocery-store display method; without them his idea was a mere opinion. And there they all lay in his own improvised shorthand--countless hours of work--out there on the ledge.

It should be noted that his findings were written "in his own improvised shorthand." The author adds this information because it serves to make the yellow sheet that much more valuable. Since everything is written in shorthand, the paper actually contains the equivalent of several sheets of handwriting. The information is all compressed. If written down in conventional prose it would have to take up more than one sheet. But if it were all written out in conventional prose rather than shorthand, it probably wouldn't be able to take flight the way it did.

Jack Finney spends several paragraphs describing the struggle that goes on in his protagonist's mind over whether to abandon the paper or whether to try to retrieve it. This is good writing. The reader would be less likely to identify with Tom Benecke if Tom simply decided to climb out the window and crawl along the ledge. Tom, like the reader, knows it is dangerous; but the reader is finally convinced that Tom made the correct decision, because the reader finds himself, in his imagination, out there on the ledge with Tom. If the reader were not convinced, he might stop reading the story altogether.

Everyone is familiar with the feelings created by being up on a great height. Some people have a phobia that prevents them from going even as high as the second floor of a building. It is very easy to identify with Tom, especially since we are confined to his point of view from beginning to end. We suffer his terror and his paralysis. We experience his epiphany when we realize that life itself is more precious than any hypothetical reward. The story is successful because the author succeeds in persuading the reader that Tom's decision to risk his life was plausible, even though it turned out to be a bad mistake.

And it occurred to him then with all the force of a revelation that, if he fell, all he was ever going to have out of life he would then, abruptly, have had. Nothing ... could ever be added to his life. ... He thought wonderingly of his fierce ambition ... he thought of the hours he'd spent by himself, filling the yellow sheet that had brought him out here. Contents of the dead man's pockets, he thought with sudden fierce anger, a wasted life.

This is the same question Tom poses to himself in the quoted text above: what was it for? The quote tells us clearly the answer: it was for Tom's "fierce ambition." Let's examine how this ambition is revealed so we can see what Tom sees so clearly finally. The introduction in the exposition of Tom's conflict will help us understand Tom's answer.

When Tom helps Clare on with her coat and smells her perfume, through the closely proximal or near narrator, Tom confesses that "it was not actually true that he had to work tonight." Tom, through indirect dialogue provided by the narrator, explains that though he doesn't have to, he very much wants to work because the Interoffice Memo he wants to type explains and lays out the details of an independent project he has been laboriously collecting data for. He wants to get the data typed up to present to his boss next day for reading over the weekend.

This demonstrates for us that Tom's ambition leads him to take initiative, devote personal time to research, and give up his personal life of comfort to pursue a long-term plan for advancement in his career (pursuing such a plan, in itself, is not an error). This explanation is the first, small picture we are given of Tom's driving ambition and the first introduction of the internal conflict of ambition versus personal happiness.

You can examine the text for other instances that shed light on the answer Tom gives. Examine the portion of text when he is out on the ledge and sees Loew's theater, then analyze its connection to Clare. Analyze the few lines Clare speaks, then see what other word can be spelled from the letters in her name. Examine the flashback narrated while Tom is staring out the window at the lost paper on the ledge when he thinks about how he accumulated the contents of the yellow sheet of paper. These and other textual instances you can find will draw a strong picture of Tom's fierce ambition and reveal how ambition drove him out onto the ledge.

For many seconds he believed he was going to abandon the yellow sheet, that there was nothing else to do. ... [His independent projects] were the beginning of the long, long climb to where he was determined to be, at the very top. And he knew he was going out there in the darkness, after the yellow sheet fifteen feet beyond his reach.