Good documentation explains that which is not naively understandable. Most production-hardened code has lots and lots of code that deals with edge cases. A new

Good documentation explains that which is not naively understandable. Most production-hardened code has lots and lots of code that deals with edge cases. A new employee might look at all the code that deals with the edge cases, think "this is needlessly complicated", and rip it out. Sometimes you can handle this by writing out a test case and document the test case instead. Other times, you can't. It's context-specific.