1) to change the coding of your instruments so the navigation instructions appear on the DIS. Also, on some models, to code the CAN gateway.
2) to check, and correct, the coding of the RNS-E so it is correct for YOUR car.
3) to clear the fault codes that are bound to appear on the RNS-E while you are installing it - usually things like bad antenna connection (because you haven't connected it yet!), intermittent communication (because you plugged it in and out a few times), etc.
Sometimes you can get away without needing a VAG-COM, but not usually.
A correctly setup VAG-COM leaves no trace for the dealer to be able to see what you've been doing!
Just thinking out aloud whilst viewing this thread, do the CAN wires have anything (no mater how remote) to do with the TV video input signal. I know the CAN wires tell the unit there's a TV option there...
I'm just trying to exhust all possible answers to my own on going problem.
I've suddenly lossed (after a flat battery which i've just recalled) my video input into the RNS-E from a Dietz. All functions of the RNS-E are fine, just lost the picture whilst in TV mode. (I get a black screen with a thin line at the bottom).
Thanks must go to Lee whos has suggested testing the RGB output, but i've no lead to test it. In fact, i've just come back from my Car Audio Installer who too hasn't been able to test it!!!