A sled do a few things for you.
You can cross-cut wider boards that won't fit on a miter saw. The sled I made here can cross-cut up to 21" wide boards, cabinet doors, etc.
You can also cut very small pieces, since you can clamp parts in place. Try shaving a 1/16 off a 1"x1"x1" cube any other way.
The kerf works as a zero-clearance insert, reducing tear-out.
With length stops, it's easy to make multiple parts that are exactly the same length (which is just as easy with a stop on a miter saw, but probably less chance of workpiece binding between blade and stop.
Properly made and setup, it will always be dead-nuts square to the blade. Hard to always return a miter saw to *exactly* 90 degrees without some checking and double-checking.