It's for nothing as in, energy is consumed and nothing of value to society is produced. When a bank uses energy to run its services, those computers actually achieve something (like, for instance, managing to process more than a handful of transactions per second). Bitcoin's 'proof of work' is just a very complicated heating system. You are literally going through the whole rigmarole just to increase the entropy of the universe. Considering the effort we are putting in to squeeze more efficiency out of everything from washing machines to jumbo jets, this is kind of waste is pretty shocking.