Well , the main idea of steem is to see it as an investment. So if you buy steem you will power up your account so your votes will worth much more. So having much power will also get you more curation rewards in the long run. But also if you buy steem and just amass it and then trade if for something else when price goes up you can be making a profit out of it. So either way you will be getting something in return for your investment.
Ok that makes sense. But where does the reward (the dollar amount we see below every story) come from? Is that not paid by the people who upvote?
Well the system is a bit different, so the amount you see in a post will be split like 75% to authot and 25% to curators(the ones that vote), the 75% will be either split 50/50 to SBD and steem or 100% Steem power. At least as far as I know that is how it works.