A well written piece of code will only tell you what it does, a good suite of tests will teach you how to use it, but only natural language documentation - be it in comments or separate documents - will explain to you why the code exists in the first place, and why it looks the way it does.
(As for "self-documenting code", unless a lot of your functions contain the word "because" in their name, the code isn't really self-documenting.)
