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.
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.