Steem power is the determining scale about how much reward you can give some one. The more sp You have the more you will be able to reward others.. But the lucrative benefit of having more sp isn't always about rewarding others.. See based on the amount of reward other gets you also get some back for your curation as curation rewards. Means the more sp you have, the more you fill your daily upvote quota the more you will get curation rewards.. Gathering sps are for the long game that visionary users of steemit have.
On the other hand steem can be used as instant liquid tradable tokens..means you can transfer steem into other accounts whenver you want. Now keeping steem stored can also be profitable sometimes. You can sell them when market steem price is high and buy them again when the price is low. Its almost like how share markets work.