As a lot of eletrical items come of the shelf for 12V, so this is logic to use 12V. But that would not require a second battery. But is also from a safety perspective.
When you stop the car a relay disconnects the 800v battery from all other electric systems (except charger components) meaning that all electric devices no longer have a connection with the Battery.
But you would still like to be able to turn on the car (Activate the relay), get inside the car using remote. This requires power that is seperate from the big battery which requires a battery or other energy storage device. Where 12v battery is the most common (cheap) one and then it's also cheap to connect standard car components on the low voltage network..
I would be surprised if this lasts for another decade when BEV become the standard over ICE..
Not sure why they not keep the battery charged constanlty when it's driving and you hear (not EV6 specific) that these batteries run out of power.
Not sure if V2L depends on the 12v battery, it would be funny to enable the use V2L to charge the 12V battery

when it is drained for some reason...