My experience is purely with my D4.

You change the battery and then 'tell' the car you've changed it.

I've always done it as I've always had a GAPII tool to do so and it takes seconds, but have often wondered if it's really neccesary.

I've seen explanations that the ECU uses the information to 'adjust' the power requirements from an older battery, which I believe to be complete B0ll0cks.

I've also heard that it's something to do with the calibration of the Stop/Start which sounds more feasible.
But as I mentioned, I've not noticed anything wrong 2yrs into the last change on my D4 without doing the ECU bit.

HOWEVER, again, that's a D4 and the L406 could be completely different.

RRV8SC wrote:

What exactly are you “coding” when fitting a new battery?

BMS reset for the new battery.  “If it ain’t broke, don’t fix it” -where’s the fun in that?

For the older stuff, at least, telling it there’s a new battery is supposed to recalibrate/reset the Battery Monitoring System to avoid overcharging etc.