Let's say music is needed for only 43 seconds of film. You have to score it so it is an entity, so it won't bother anyone when it ends so quickly. Or if a song runs 2 minutes and 45 seconds, but the titles run a minute longer, you have to arrange that song so it doesn't get repetitious.
“They spend an awful lot of money on I-don't-know-what in Hollywood movies; I certainly didn't get any of it. But they sure do love spending money.”