The author Jack Finney provides a detailed description of the paper's movements in order to show that it hasn't blown away but is tantalizingly close to Tom's apartment window. He watches it until he realizes that it has finally come to a spot where it will remain permanently, or at least for a long time. He had been hoping that it would blow off the ledge and fall all the way down to the street.
He knelt at the window and stared at the yellow sheet for a full minute or more, waiting for it to move, to slide off the ledge and fall, hoping he could follow its course to the street, and then hurry down in the elevator and retrieve it.
Finney wants to establish that there is no way for Tom to retrieve his paper unless he climbs out on the ledge. Tom can't reach it with his arm, or with the poker from the fireplace, or with a broom or mop. But at least he is assured that the paper will remain where it is if he does decide to take the risk of climbing out on the ledge to retrieve it. At least he wouldn't be risking his life and then having the paper blow away just as he was about to grab it. He knows exactly where it is and exactly how many steps he would have to take in order to get to it.
Tom is upset about the loss of this particular piece of paper because it contains notes he has taken for a proposal which he thinks could win him recognition and even a raise and a promotion.
The work could be duplicated. But it would take two months, and the time to present this idea was now, for use in the spring displays.
It is significant that the sheet is covered with notes on both sides and, furthermore, that they are all "in his own improvised shorthand." This means that if they were transcribed, or if they had all been written in longhand, they would cover several sheets of paper. Finney deliberately mentions the shorthand to increase the value of the paper to Tom as well as to show the reader how much work he put into it.
Accidents like this have happened to many people, and the reader can appreciate what a maddening thing it is to have to go back over work that is already finished and has cost countless hours of hard work. The author emphasizes the value of the paper in order to explain why Tom will finally rationalize doing a crazy stunt like climbing out of his eleventh-story window onto a ledge that isn't even as wide as his foot.
It occurred to him that if this ledge and wall were only a yard above 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.
It is his own imagination that is his worst enemy. If he can only pretend that he is just a yard above the ground, and if he can resist the perverse temptation to look down, this little adventure will be a cinch. It might even be fun.