Why does Tom tell Clare that his work has to be done tonight?

Tom's a workaholic. Work has taken over his whole life to the exclusion of all else. He's become completely obsessed with getting promoted at work, and he feels that the in-depth research he's been conducting is essential to his chances of success. That's why Tom has to stay at home to do some work instead of going with his wife Clare to the movies. He feels bad about it, to be sure, but he still puts work ahead of his wife and remains at home, frantically scribbling away on his yellow paper.
This is a man who clearly has his priorities the wrong way round. He needs an urgent lesson in what's really important in life. It duly arrives when his precious sheet of yellow paper is suddenly blown out of the open window and onto the ledge outside.