Sweat guards are mostly for the rear of the slide, not the frame. The slide is steel.

The steel against flesh can be uncomfortable
The slide finish can wear—rust
The sweat guard helps guide the pistol into the holster
SG helps keeps clothing away from the mouth of the holster
It keeps the rear of the slide from abrading underclothing
Rear sights can have sharp edges and corners. A SG prevents snagging
There are other personal reasons I may have missed.