This is something my wife asked me tonight.
There are a few reasons:
Between family, work, and 3 first gen cars I am getting disorganized and forgetful. Mike Labrecque sometimes asks me to look for info or for replacement parts and a few times I have forgotten about stuff.
I would like to keep a record of information that I find out. Little things regarding what is correct on a 69Z can drive me in circles for a long time until I figure them out. For example, the silk-screening on the underside of the air cleaner cover. For a 69Z is it supposed to be curved or straight text. Weird and unimportant question? Maybe, but for a judged car, maybe not. On my rusted out 69Z cover that I believe is original, the text is straight. A bunch of guys on the CRG say it should be curved. I found out from a trusted source that he has seen these covers both ways on a 69Z. For now I can stop going in circles.
The car has a lot of history to it. When I talked to the former owners, I can tell that the car meant a lot to them and played a big part in their lives. That alone makes me want to document and keep the history on this car.
That is all I can think of for now but I am sure there are additional reasons.