How Does Steem Power Grow Over Time?

In one of @adil's posts, he mentions the incentive of powering-up: "Steem Power Grows in value as Steem Coin value Grows on the Market".

Can someone explain how this works exactly? I understand that Steem power is useful for increasing your vote influence, but aside from that, I still don't understand why it's better to lock-up your Steem into Steem Power, when there does not seem to be a posted interest rate?

Does Steem Power convert to more Steem over time? If so, then that would make sense.

Please enlighten me. Thanks.