I think what you are questioning is, "how come I can trade my Steem for BTC?"
Well, because, much like the USD, people are willing to accept it as a means of exchange, there is a belief system.
Soon as that belief system no longer exists, there is no value in Steem. Much like the USD. The USD is only 'worth' the paper it is printed on. The belief system behind the face of that dollar bill, is what makes the note valuable. The belief and trust, that you can transfer it in exchange for another commodity, or service.
At least, this is how I see things. :)
Kind regards,
Optimistic