And to be perfectly honest, well written code might not stay well written as project circumstances change. Thus, documentation that can be updated often makes life easier going forward.
And even worse, sometimes code can't be well-written once you start interfacing with third party software with quirky shenanigans. We got quite some comments flying around such as "In case you are wondering: yes, this call is necessary, or this and that situation will propagate through these two libraries and break with this obscure, unhelpful error message. If you ever change this, look at those documentation files for unobvious, scary testcases to look at".
