Question

why do many supermarkets and other public places have ramps

Answer 1

Ramps in the public places are made for some special people. Those special people are the one who can't walk and are on the wheel chair.
Ex. Stephen hawking and many other people who can't walk.

Answer 2

Ramps are used to provide access between vertical levels.They provide an alternative to stairs for wheel chairs users ,people with mobility issues, etc.