Some of the other communities I'm a part of on LJ require certain things of new members' accounts (for example: a certain comment count, member of other communities, recommendation by current members, etc.). What this does in those communities is prevents assclowns with sockpuppet journals from getting in and trolling, trying to get the comm shut down, or generally causing trouble. It is a terrible amount of work for the mods, though. Still, if people are posting from their real, primary LJ accounts, they're less likely to attempt hit-and-run douchebaggery without fear of consequence both in the current community and others who might hear of it and decide to ban the user for the same asshattery.