Whats the point of clans anyway, it doesnt really confer any bonuses to you, and there are not any clan rankings either, so what is the point of even being in one exept having the right ot change your screen name.
You can't have a real clan scene with a competitive ladder until you have a couple organized clans first. Also, even without a formal ladder or league, sometimes clans will play against other clans.
In case you have no idea what a competitive ladder looks like, here's an archived webpage showing teams ranked on a ladder. Why have clans and why play in a formal league? Because formal clan matches are much more challenging and intense than spontaneously-formed games.