I think coinbase is doing this mostly to create the economy. With USD, 1 dollar = 1 dollar but with USDC, they can print it based on how much liquidity is present in the ecosystem. Kind of like how if a bank has total deposits of 1 million, it doesn't need to hold 1 million in bank vault all the time. This seems like a way to add more liquidity in the system then thr really is.

Second thing i can think of is probably how tax laws will operate in future. If you sell for dollar or if you traded one asset for another.

Mostly stable coins were created to get around the problem of bringing money into the system through banks as all exchanges can't connect to banks but for coinbase to get into this even though they have banking partners is quite weird.