The way that I understand it, Steem Power is used to give people an incentive to keep creating and curating quality content. The more Power you have, the easier it is to have your content seen and distributed. An analogy, if this were twitter--SP is more like the number of followers you have. So by building power, you have a higher stake in the gaining of future currency that is released.
RE: Still Confused by Steem, Steem Dollars, and Steem Power? The Power Plant Analogy