It simply looks better. But there’s another way of looking at it: many garments featuring garters simply cannot or can hardly be worn with the panties over the garters. So, just because in some instances you can (with usually quite basic, “thin” garter belts, worn on the hips), doesn’t mean you should.

Some illustrations of the (virtual) impossibility to wear the garters under the panties:

Get it?

Source: Boss Ms Becky