I just read an article by @steemgoblin that explains why it is important to promote your quality articles outside of SteemIt. In short: quality posts promoted elsewhere will attract more users, which will increase the demand and price of Steem. All this will result in higher rewards for your posts.
You can read the article here: What if You Could Help Raise the Prize Pool to 1 Billion $$ DAILY? How Quality Content, Aimed at OUTSIDE Audiences Could be Making us ALL More $$$$