its a good question, and i think the answer lies in when you add dollars to coinbase, there is like10 days seasoning before you can withdraw it back out. so if you swap into usdc instead of withdrawing, you dont have to wait for it to season in  your bank, then deposit it back into coinbase. its a tool for people that trade shitcoins within an exchange like Coinbase's ecosystem that dont want to deal with delays relating to pegging in and out of fiat.