Having steem is something that can be traded aswell like SBD but SBD has more value in bitcoin form. You can look up price comparison for the both...
But powering up your steem can help you get more voting weigh... the more steem the more your vots is worth for starters when you start off you may get 0 to 1 cent per your vote but if you get more steem power (when you power up your steem) your increase in money per vote goes up..

Search in steem... "What is powering up" "Powering Down" "Difference between steem and steem dollars"
Your steem stays in your accoutn for aslong as you have it but powering up keeps it on hold until you "power down" to cash it out on other exchange sin to BTC or whathave you but it takes 13 weeks to do so but every week you get a payout from powering down.

Idk i may have confused you but just look up those terms i said and you should have the general idea :)