I think "speculation" is the one of the ultimate reasons for ALL of the digital coin prices, including bitcoin. Also, all stock market prices are based on speculation. So comparing the speculative price of solarcoin with the speculative price with many of the other altcoins, then the price of solarcoin appears very cheap right now... especially in consideration of the extreme potential this coin has to greatly help improve the renewable resources sector of energy. Thus, some people don't see an investment in this coin as just a "speculative" investment hoping that it will increase in value on the exchanges for a quick (or even long-term) profit, BUT rather many of the investors in this coin are actually investing into the future of the global environment by supporting the production of solar panels.

The "use" for this coin comes naturally with increasing popularity. As there are already companies in Europe who will accept solarcoin in exchange for products/energy, such as ekWateur. See this article for more info:
https://cleantechnica.com/2017/06/29/solarcoin-cryptocurrency-earned-generating-solar-electricity