Because the whole premise is that you don't really have a way to store or use that energy in other ways. Then converting it into cryptomoney makes sense- then you can use that crypto to buy things which are helpful to the local community- even things that are produced on the other side of the world, where your energy could never reach.

An interesting thought is that of using that crypto exclusively to buy energy when the prices are non-negative. But I'm not sure that makes sense from an economic point of view, it's a snake biting its tail.