This popular wireless mouse from Microsoft has a little issue - from time to time the mouse wheel feels like it's gaining a kind of rubbery resistance.

The wheel rotates, and by design it's allowed to roll from side to side as well. This side to side motion means it has a longitudinal shaft. After some use that shaft loosens so that the rubbery mouse wheel rubs against the body when rolling the mouse wheel away from you. Feels like a resistance, and doesn't feel 'nice' any more.

This tiny spacer goes between the shaft and its mount to remove the longitudinal movement of the shaft, preventing the rubber wheel from rubbing on the housing and making your mouse feel new a fresh. And who doesn't like a fresh mouse :-)