I think you have answered your own question, the reason they want you to enter every last detail is so that they can fill in the gaps.
This will only have to be done once, as next year all the details will be on file for its next MOT
After a couple of years virtually every vehicle should have been through a computerised MOT. Siemens can then sell this data to who ever requires it, such as motor factors. When you phone up for parts they will type in the reg and every detail should be available, hence less wrong parts being sent out
As you enter failures, all the parts that the vehicle fails on will also go into a database. Motor Factors can then use this info to tailor their stock requirements (keep more of the popular failure items)