Hmm this is some deep existential stuff...
Seems pretty obvious to me. Why multiplayer? Clans (or guilds, corporations, player associations, or whatever you want to call them) are simply a natural progression of the multiplayer aspect of pretty much any online game.
In a game like Wargame, teamplay is a highly rewarding experience. Playing with the same team repeatedly allows you to develop a relationship and a trust that you don't get with random teammates. Sharing the glory of your victories and the pain of your defeats with your friends helps you get a sense of esprit-de-corps and camaraderie that you'll never feel if you always play solo or with random pickup groups.
It's a shame that Eugen dropped the clan tag functionality of W:EE and never iterated on it. More in-game support of clan functionality would be awesome. Thankfully we have a pretty vibrant and competitive community regardless.