A log like that is useful for yourself if you have a dozen or so AEGs and pistols.
I keep notes from when I chrono, what BBs, and so on.
I mark scope and accessory locations. Number mags, and so on.
When I change parts I save the old ones I take off in the new packaging.
I print out exploded views from the net and make notes and save all the manuals that come with any airsoft part in one place.
All that is for me though, I don't share it since airsoft guns are inconsistent so something I've found might not work for another person.
Airsoft, where nothing is hurt but feelings.