Clans are for friends.
I'm going to go against the crowd here and disagree with this. Mainly because clans have taught me a great deal about building. And although you can learn those skills on your own, clans seem to help you achieve that skill better.
It's also a great way to meet people in the game, and get connections as well as online friends. But it's also the fact that some of the best builders in Blockland are in clans.
So it just goes to show you, clans aren't for "friends".