A through booking provides protection in case of delay and an entitlement to be rebooked onto another flight should a problem occur.

Holding 2 bookings on 2 flights just in case, is a peculiarity that AA sometimes has provided

The agent in London was quite correct in offering the option to confirm the later flight and cancel the earlier or to gamble on the eariler. If you took the gamble and lost, the airline would still have been responsible for rebooking you onto a later flight