Good question!
When you exchange STEEM for Bitcoin, you are pushing STEEM exchange rate down, because the exchange market now has more STEEM supply.
When you buy something for STEEM, you exchange it for real value: a good or a service. It will even push the market to buy STEEM in order to acquire more goods and services.
Also, as we create more ways to #UseSTEEM, we build a chain and eventually the full loop, so less STEEM is circulating in exchanges, influencing the price up :)

-- @develcuy