If a budget isn't spent, in a lot of cases it will be allocated elsewhere, so to maintain the budget, it all has to be spent.

ELI5 - If you're working on a craft as a group and the teacher gives you 10 glue sticks, they might notice you only needed to use like 3, and give you less next time because they realize they have you too much, and they can give it to another group that needs more glue. If you want to keep all the glue, the one way to show you needed all that glue is to use all the glue.