Powering up your Steem allows you to give great content on Steemit a higher vote. This in turn benefits you by giving you curation rewards, in time, growing the amount of Steem you own.
IF you leave your Steem as just Steem, it can't be used to vote for others or grow the amount of Steem you have. Unless you are just speculating or trading Steem, If it were me, I would power up most and save some for certain emergencies or even vote buying. Then again, that's just my two cents =)