Well a bank account provides all the functions of money, and to a much greater level of functionality. But the whole reason cryptocurrency exists is to avoid so

Well a bank account provides all the functions of money, and to a much greater level of functionality. But the whole reason cryptocurrency exists is to avoid some of the issues you get when dealing with centralized financial institutions. If you want to decentralize that role, you need a new way of managing scarcity. Proof of work is one way to do that. Whether that’s wasteful or not would be a personal judgement, but the reasons the demand exists are pretty simple to understand.