Powering up Steem allows you to delegate to projects and earn passive income through SMTs or Steem from a part of the share in profits.
For example I get Magic Tokens by delegating to Magic Dice and earn Steem passively each day. I am hopeful that if I want a quick return one day I can sell my Magic Tokens to someone else looking to earn passive income through the site.
another awesome point I never thought of - it gives you the ability to delegate to other projects