Cryptocurrencies have a value because people want to buy it. Why would people want to buy Steem or SteemPower?
A hope that the price will grow even more, and that one ultimately will be able to sell for a profit and buy a big bread or a yacht (YMMV) later. Most of the value for most of the cryptocurrencies come from this. In itself it's unsustainable, any cryptocurrency where the demand is only driven by speculation, the price will bubble and then collapse. Steem is quite unsuitable, due to its hyperinflationary nature. SteemDollars are also completely unsuitable as they will never grow above the USD in value. SteemPower is more suitable, but not the best since it's difficult to convert it into a yacht (however, it does have the potential to support your daily intake of bread).
For Steem, there are some specific reasons to be optimistic:
1a) People may have a strong belief that the SteemIt platform will grow due to user adoption, and hence the Steem Power price will grow.
1b) People may have a strong belief that the Steem blockchain holds more potential than the SteemIt solution, and that the SteemPower value will rise due to this.
1c) Some people have a hope that Steem will succeed as a payment token. I have my doubts on that one.
Some people may buy Steem for no other reason than to support content creation and creators on the Steem blockchain. This is a very legit reason for buying Steem. Others may buy Steem to support the SteemIt platform.
I believe quite a lot of people buy SteemPower with the hope that they will gain power and influence in the Steem network that way. We all have a dream of growing from minions to whales, don't we? I must admit, I feel a bit like that - a small press on the upvote button on my own comment, and ... oups, it's at the top and not the bottom of the comments listing. People will be gratious if an upvote from me actually contributes to a meaningful amount of money. Maybe I'll get more followers. This "power greed" feels a bit politically uncorrect however.
I believe that most often people buy in due to a combination of those three; each and one is not strong enough on its own to spur demand for Steem.