Key off battery power to an OEM ECU is traditionally used for volatile memory functionality, been in place since EFI day 1. If the ECU doesn't utilize volatile memory or some other key off function like alarms etc. there's no need for it.
Traditional volatile memory items- fault codes, learned values, OBD self test values, etc. There's a LOT of learned/adaptive stuff going on in OEM ECU's even from the late 80's/early 90's.