arhag's (currently #1 ranked) post summarizes some of the thinking, and also I'd suggest my comment there for a bit of elaboration. In essence Steem Power is a prepaid subscription to network access. By stocking up on Steem Power now you are speculating that access to the network will be worth more to more people in the future, who will then be clamoring to buy tiny slices of your Steem Power to get in or to maintain their access. Obviously, that is a very risky bet (all cryptos are, including Bitcoin).

https://steemit.com/steem/@arhag/where-does-the-money-come-from-a-look-into-the-economics-of-steem