It’s not that you end up with good documents, it’s that similar effort results in more useful documents. Pasting a long conversation into a document kind of a pain to work from, but it’s fast and a much better starting point than a vague description without any real context. The next person reading the document might be motivated to clear up any misconception they had, where starting from scratch seems like a huge effort.

PS: The second bit is surprisingly effective. I have created a few bare bones documents that ended up becoming very fleshed out in a year+ when some sends it back to me.