Hi Roy, to answer "Anybody should be able to go to a breakers, buy a part for their car (obviously same model, same part number) plug it in and it should work. Why not?" The reason for coding in simple terms is to tell the unit being fitted what other systems are fitted to the car PRESENTLY.eg you get an engine ECU from the scrappy,same model,same part no but the car it was on had ABS no TC,was an auto and two wheel drive.The car you're fitting it to has ABS/TC,is a manual + four wheel drive.As you can see this will cause problems.
So why not self learn I hear you say,ok fit the unit + one of the systems is not working,the unit codes as this system being not present and when this gets fixed then we have a problem.The other option would be to order a new ECU from the dealer to the chassis no which comes coded to the car,which would take weeks and would not work properly if someone had retrofitted any extras to the car since it left the factory.So the units being codeable leaves them universal and therfore cheaper + reuseable if them can be recoded.(in my opinion)
Hth Sean