Steem power gives you influence over who earns with your upvotes. Someone with 1000 STEEM Powered up will give 1 cent for an upvote. The more in your wallet the more value you can give from an upvote, there are some accounts worth hundreds of thousands and even millions (Whales). A single upvote from them and you can get tens of dollars, doesn't sound a lot but if lots of people upvote your post you can earn a decent chunk of change. You could buy STEEM from an exchange and power it up to vote for other people so that they earn , you could also vote for your own posts and earn that way too. If you buy the token it can appreciate in value the same way as BTC. Buying any token is dependent on individual circumstances, only invest in what you can afford to lose. Buying USD or Diamonds might seem like a good idea but USD is declining and diamonds are made from the most abundant atom on the earth. Buying BTC is also risky, sure it can go to 100k per coin but it could also collapse if a bug is found or scaling hits a deadlock and it falls from the top 10. I think an account of 10k Steem gives 6 cents an upvote. This can change based on the price of STEEM and probably what is in the reward pool which is used to payout the STEEM and SBD.

BTW I am not an expert I don't pretend to know the mathematics behind the algorithms just as I don't know everything that is going on under the hood of my car when I drive it :]