You are getting the actual build information specific to your car, from day 1 in the factory. Engine unit number, options, all the information that would prove how your car came from the factory. The dealer code is on there, you would have to cross reference that to other information on this site. (I think there are still a few dealer codes that have been lost to time, at least the last time I looked) due to dealers that closed many years ago.
You do not get the car's build sheet, those would originally come inside the car somewhere, and may have been discarded (or made fine mouse nests) in the past.
In my opinion, the rarer the car, the more valuable it is. It is awesome to have a service like PHS for our 'birds. Chebby' does not have anything like that, thus there are thousands more Z28's around today than were ever built, and hard to prove which are which.
Tough to prove your 'bird is an ultra rare, R/A 4 car when the PHS shows it was a 350 automatic....
I remember back about 6-8 years that there was a guy on this site trying to sell his '69 'ultra rare '70 concept car' when the PHS proved it was a standard 350 coupe in a lovely brown color!