What functional reasons would someone buy steem?

From what I can see, the major reason why someone would trade steem for something else would be to advertise or promote on it. Quickly gain vote power (and thus visibility) by buying steem power and using it to vote up your things. You get targeting somewhat through tags. In addition, you can accumulate a large amount of steem power and then rent out your voting power to interested parties.

That and account bootstrapping are the only reasons I can see so far.