In VAG cars, the ECU and dashboard are often paired to each other for anti-theft purposes, since the immobilizer is part of the ECU itself. To stop people from simply pulling the ECU and replacing it with one keyed to the immobilizer key chip they have - the ECU is paired to the dashboard, which often requires a lot more time and effort to remove.
For other modules they're often used across many different cars/engine/transmission/body type and regulatory markets. "Coding" a module tells it what accessories/options it's connected to, what else is in the car, what regulatory market it is in, and so on.
There's also "coding" for how the module behaves with certain features. For example, body control modules on US cars won't allow you to use the remote to roll up the windows, but by flipping one of the coding bits, you can enable it. There's also silly stuff like enabling / disabling gauge sweep on the dash.
(Explaining this mostly for others, you seem aware of at least some of this)
Yes I work in automotive and have routinely replaced ECUs. I work for an American OEM, I guess we don’t pair ecus to each vehicle. Unless its ecm/tcm/radio/security gateway and probably the key fob ecu. Everything else is just a part number that you can easily swap out. The few screws holding the controller and the wiring connector(assuming the controller is easily accessible)…
Though I own an Audi and use VCDS I was not familiar with pairing an ecu to the vehicle, however I was aware of coding in the VCDS. (I have not yet needed to swap an ecu on my car)
For other modules they're often used across many different cars/engine/transmission/body type and regulatory markets. "Coding" a module tells it what accessories/options it's connected to, what else is in the car, what regulatory market it is in, and so on.
There's also "coding" for how the module behaves with certain features. For example, body control modules on US cars won't allow you to use the remote to roll up the windows, but by flipping one of the coding bits, you can enable it. There's also silly stuff like enabling / disabling gauge sweep on the dash.
(Explaining this mostly for others, you seem aware of at least some of this)