[causi]

But what's the use in that "useful"? When I pick up a sock and it has holes, I throw it out. When the drawer gets empty enough it can fit another package, I buy one. What utility would come from worrying about when I used a particular sock?

[toast0]

It can be hard to find the same model sock when you need to restock. Wear leveling and replacing them all at once eliminates difficulty with matching. Also, if you don't wear level, you may have one new sock and one worn sock, and that can be weird.