there are 3 reasons you should convert your steem dollars into steem power 1st security, if some one was to hack your account they could instantly transfer your steem dollars to there own wallet, 2nd voting power, the more steem power you have the more $ you will get for upvoting your own posts and 3rd interest, you will gain a small amount of steem power back as interest just for holding steem power, i hope this was helpful :)