Like other anons have said, it's just to make sure random people don't join the comm and start making entries! I had an awkward experience with that once (two people I don't even know joined my musebox and made entries, though they weren't RPing), so even if I don't mind my RP partners using alternate accounts (like, icon mules or whatever) I'm not really keen on the idea of anybody who stumbles across the comm being able to join and make entries, view things that are locked only to members, and things like that, you know?