Slippers, like all footwear, were originally made to protect the feet.
Later, as various types of footwear became specialized and formalized, slippers were designed as mainly indoor footwear which could easily be slipped on (hence the name) on arrival home, or in a building, to cover the feet once the outdoor footwear had been removed.
Outdoor footwear, in times past, would become extremely dirty owing to lack of paved roads, horse and other animal transport, and lack of sanitation.
So outdoor footwear wasn’t suitable for wearing in a relatively clean environment which people wished to remain clean.
Indoor areas where people didn’t normally take off their boots and so on, such as taverns and entertainment areas, would usually have their floors spread with straw or sawdust, or other dirt-absorbing material.