Memory is not a maintenance system
The useful details disappear first: the exact measurement, the part number that almost fitted, the setting that fixed the problem, and the small mistake that added two hours. A build log keeps those details available when the job returns six months later.
A useful record is written for the person who has to diagnose the system next—including future you.
Record decisions, not just purchases
A parts list says what went in. It does not explain why that option won, what alternatives were rejected, or which compromise was accepted. Those decisions matter when the next modification changes the constraints.
Make the next attempt better
Good documentation is not paperwork after the interesting work. It is part of the engineering loop: observe, change, test, compare and improve. That is the standard this site will use for car projects, home infrastructure and hardware builds.