Because Twitter owes more in interest on the loans he took to buy it than it makes every month and he needs to dig himself out of a financial black hole.

All commonly used forms of energy storage have some efficiency loss. Pumped storage is not perfect but my understanding is that it usually comes at a 10-25% loss, which isn't all that shabby all things considered.

Pumped-storage hydroelectricity is an old and proven method for load balancing intermittent power sources. Would like to see more of that as geography permits.