The value you see under a post is based on the 7 day moving average of what Steem is worth in the real world on the cryto currency exchanges. Yes you can convert the steem out to an exchange and then sell it for Bitcoin and convert that Bitcoin into real USD currency. What you earn as Steem Power can be converted into Steem as well but it takes longer to withdraw and sell on the open market. So in essence it is and has real value, that value is determined when you actually sell it.