I assume that you are asking about the story "Contents of the Dead Man's Pocket" and so I have moved this question to that group. Please be sure to tell what story you are asking about when you ask questions...
There are two related reasons why Tom values the yellow paper so much. First, he has worked for a long time on the stuff that is written on that yellow paper. So it represents a lot of effort on his part and he would not want to lose it. Second, it is (he thinks) his chance to get a better position in his company and to get richer. He believes that the information he has gathered on that sheet will allow him to convince his bosses about a new way of displaying goods. This will make him a big shot in his company. Here is a quote that shows these things:
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.