I've got architectural plans for my house. Parts of them are useful, but most of it isn't because the house doesn't match the plans. The details on the plans for the parts that match aren't trustworthy, because of all the parts that don't match. This is a relatively new construction, with minimal remodeling; it's just as they were building, they decided to do something else, and not update the drawings.
This is different than commercial work where in addition to the original plans, you also get as-builts, which can be expected to be accurate, and are expected to be updated.
If you only want to document once, it makes sense to do it once the thing is built, rather than before, because there's a good chance the actual thing will be different than the plan. If you will update it, it might make sense to start documentation before the thing is built.
Of course, if you never get around to writing documentation, it never needs to be updated.
This is different than commercial work where in addition to the original plans, you also get as-builts, which can be expected to be accurate, and are expected to be updated.
If you only want to document once, it makes sense to do it once the thing is built, rather than before, because there's a good chance the actual thing will be different than the plan. If you will update it, it might make sense to start documentation before the thing is built.
Of course, if you never get around to writing documentation, it never needs to be updated.