There are certain issues with wearing slippers that can arise in some situations. Some people also find slippers to be a type of safety footwear if they are the large overstuffed novelty variety because the stuffing can preclude injury from stubbing one's toe while walking in a dark room at night.
Another form of safety can come in the protection from pathogens on the walking surface. Wearing slippers can be used as a way to keep feet clean.
There is also a sizing issue, as novelty slippers do not usually come in individual sizes, merely a sizing guide (e.g. 9-12). This can cause issues when trying to find a pair that fit properly. Also, the sheer size of the slippers can cause issues when walking.