A few models have free turning mouse wheels, but why do most mouse wheels have a notched mechanism?
The original mice with wheels from the 1980s were "freewheeling" and highly annoying.
Once you had scrolled the document to where you wanted, you'd let go of the wheel and—invariably—the document would jump up or down a little bit (for example, a few lines) as you released. So then you'd try to move it back to where you wanted and carefully release and it might still jump a bit.
Someone brilliantly figured out that adding a detent (like quality car stereos) completely solved the problem. This was such an easy and obvious solution that it is widely used.