Still doesn't answer why you would need any crypto here. Why can't the USD transferred to stripe just be a record in an SQL database saying customer X has N USD in the account, and transferring that around could be done instantly at zero cost by changing an sql row.

There are all sorts of protections around who can be a custodian of someone’s money (for good reason)

However there are use cases like running a marketplace, where the platform would like to be able to direct the flow, maybe hold things temporarily in case there are multiple transactions or to split a transaction up between different clients, before paying it out daily or weekly as a lump sum. Often it’s just to avoid fees, because the marketplace operator charges their fees in a different way (like a flat monthly invoice) and they want to assist with money logic as a service, but not be the custodian of the money.

Even just knowing that money has moved at all can be useful, without any ability to touch it, and it’s difficult to get permissions from conservative financial institutions, whereas permissionless ledgers make it easy.

Crypto can help add that nuance. It’s still your money, but you can give a third party the ability to do some things to assist you, without giving the ability to transfer it all to themself and run away with it.

That sounds like banking or payment processing. Albeit with later Paypal has proven that you do not always need to return funds, but still there is regulatory history on that...

Stable coins are new enough and have not catastrophically crashed yet so there is less oversight.