There are software dependancies between different ECUs, and actually loading software into ECUs is now needed regularly - and thus so is up-to-date diagnosis equipment and up-to-date firmware disks too. These cars are really very very complex, and unlike your laptop, if something goes wrong can result in fatalities.
There are too many configuration items in every ECU now to have someone set options by hand whenever an ECU is changed, or to understand how changes on one device have to be done on other devices. The move to SCN coding was *very* important.
