My cat growls when a car pulls into the driveway or when someone knocks on the door. She is a guard cat!
They bring us endless love, joy, and entertainment!
To be more specific, she is an alarm cat. Any serious intruder would not be deterred by a cat.
My dog does the same posturing at anyone who comes on "his" property. But when anyone enters the house, he's all wagging tail and licking tongue. He's a good alarm, not a good guardian. But I can live with that.
My cat...well...he just runs and hides whenever anyone comes to the house. He'll check in later when things settle down.