It's a gamification of rewards for content creation.

That's what I (naively) believed when I joined. And when it works, because of good people creating good content, it works beautifully. However, the gamification rules are weak, and spam authors can easily garner a lot of steem power. Which is a serious problem; even much older systems (such as email / blogs) requried spammers to spend some cost / effort.

Now, one might say that it doesn't matter.. we could just ignore the spam and keep steeming for content and treat rewards just as a side-effect. But then, why would one use STEEM at all? One could simply use an open-source social network such as mastodon along with a system of direct rewards instead of indirect rewards.

All that said, I am still hanging in here for a few more months, hopeful that a solution to spam is found soon.