Documentation-writing is not a goal in itself. Code, tests, documentation – all of these are tools to communicate in writing with other programmers. That includes your future selves as it includes teammates and project collaborators. Kent Beck, “The Documentation Tradeoff”:
Why change perfectly servicable code today when there are problems to solve? “I’d be changing it again next week. So where’s the value? Might as well keep it as-is.” When you are inclined to change code without defects, this could indicate a new understanding that needs expressing. Changing existing code to reflect a new understanding is all we have. This is how we pay off debt in our design.