There are many unique characteristics of pet doors which do not typically apply to general doors. One attribute is restricting use to a particular individual animal or maybe an entire species merely by their presence at the door, and without requiring an entry combination, key, or intelligence of the critter.
Residences might use a pet door to allow movement between indoors and outdoor at will but at the same time preventing burglars, wildlife, etc. from getting in.
Often pet doors are installed in a people door. But they can be installed in a wall, too. A smart pet door could prevent this.
Modern cat flaps are popular in the United Kingdom where it is estimated about 74% of cats have access to the outdoors.