Well there's alot of reasons. For one it keeps the cool air in on hot days like today. Then of course you can lock the door to keep people out while you're sleeping. Think of it like having a rock in front of your Den that you could move from your side but someone outside couldn't.

[Rita leads the way inside while Beastboy explains the concept of doors with an amused smile on her face. It's not the first time he's had to help a non-human adjust. He seems to take great pleasure in it though]