From this post you will get rewards in Steem and Steem Dollars, both of them have no utility other than being able to send them without fees and in 3 seconds to anyone with Steem account. That's neat in itself, but you could power up that Steem which will let you allocate some of the daily generated Steem as rewards for content creators by upvoting.
For example, with current prices having 5k Steem Power would mean that you could reward 10 different people here on Steem with about 1,3 dollars a day. And if the prices stay the same, you could do that forever. With that much Steem Power, you would get slider which will let you make smaller upvotes as well, enabling you to make more votes than just 10, they will just have little less impact.
So in essence, Steem Power are shares to Steem blockchain that let's you have more say on who should be rewarded here and who not. And as more people join here, and demand grows for visibility, the price of Steem should rise. Meaning your votes will have larger impact and the wallet in total will be worth more, as Steem Power is just Steem that is locked up and that can be powered back down to Steem and sold.
I hope this clarifies the system little bit. Ask away if you have any questions unanswered.