Physical systems aren't perfectly predictable, and so require feedback. You include a little more fuel than you think you need so that if you use more than exp

Physical systems aren't perfectly predictable, and so require feedback. You include a little more fuel than you think you need so that if you use more than expected, say because solar conditions make the atmospheric drag a bit higher, you won't run out with the payload just shy of the intended orbit. But during re-entry there's no reason to keep that with you and if by some chance your stage arrives intactish at the ground it might cause a fire. Hence the venting before re-entry.