This is a little like a system which says "we will use all free space behind your filesystems linear maximal extent as swap" and then discovering it gobbled all your freespace and DF reports you have no space. As long as the space comes back when you need it, you don't have to care. But, time machine, and swap both have a habit of grabbing stuff at high privilege you can't always directly impact without using abstruse methods. So, in practice while you can say "it doesn't matter" it can matter, depending: it can cause dismay, and it can interfere with your otherwise normal use of the machine.

"free" space being contextualised isn't entirely nice. better to be clear how its consumed, than say its free.