It bothers me to see pet cats roaming freely outdoors. Their parents most likely think this provides them with mental and physical stimulation. And it does. A cat can certainly have fun outside playing, hunting, chewing grass, exploring its territory, and climbing trees. 

Your indoor-only cat must occasionally leave its house. It should visit the veterinarian at least once a year. It might also need grooming. You might move to a new house or apartment and need to transport your cat there.