It's easy to accidentally accumulate more than you need. E.g., when I got married we each were previously living our own lives. Big things like mattresses we consolidated, but an extra set of sheets is zero maintenance.

It's also nice to be able to make the bed right away. Maybe you want to take a nap while the clothes finish. Maybe you have time now and not later (and can defer the folding to "later" as well). You could have one of those shitty high-efficiency dryers that incorrectly detects whether your clothes are dry and doesn't have an override to apply heat anyway, requiring 12h of air drying to actually have dry sheets. You could not have a dryer or otherwise prefer air drying more generally.

You could also have an extra set of sheets for a pull-out couch or other sort of place where you don't normally waste the space having the bed always down on the ground.

Etc. Life is complicated. There are 8B of us, and we're not clones.