Why do dogs do this?!?

Dogs are territorial, so they like to let people know that this is their domain, and of course, the only way they can do this is by barking. They also pick up on the signals you send, so if you're relaxed and welcoming, then so are they. I expect if you were less accommodating - maybe if the caller was a sales person or a missionary - the dog may continue to growl or bark. However, they also bark when they're pleased to see someone, so it's a question of listening for the tone.