Crypto is a tradable asset, and like any other tradable asset you can make money with it. In other words you can make money with money, just like a bank, only decentralized. It's not about the technology of the banks or crypto per se, but the primary function of what banks actually do, which is save or lend money (really both).

If you think crypto currency was created to buy hamburgers, or make your energy bill more efficient to pay, as the saying goes " you can't see the forest for the trees". When was the last time you used bond certificate to pay for dinner?

The fact that people are making money and lots of it in crypto using traditional financial instruments is all the proof you need to know that it has a use case.