STEEM powers the platform, it determines how strong your vote is for posts and witnesses. When you do a post, half your rewards are paid out in STEEM and half in SBD (STEEM Backed Dollars). SBD are supposed to be pegged to the US Dollar, but recently there has been a surge of dumb money buying SBD at 10-13x the peg.

When you sell your SBD on Bittrex for Bitcoin and buy STEEM with that Bitcoin, you are taking Bitcoin from outside the platform and investing it in STEEM -> boosting the amount of money flowing round the platform. Where as if you just sell your SBD on the internal market, no new money is coming in XD

The higher the price of STEEM, the more post rewards you get for each post. What you will notice is your pending posts will go up and down in value with the price of STEEM.