It's similar to a company issuing stock. The stocks don't have value until investors choose to purchase them with money. If people speculate that the company will become more profitable over time, the value of those stocks increases. If people see more potential in Steem as a Reddit-like service, they'll pay more for the tokens. Those who "invest" them as Steem Power (instead of speculating on them) earn interest, like a stock dividend. Eventually the tokens may actually purchase goods and services, because they represent valuable content, and can be used to influence the Steem network. It's quite an interesting concept actually :)