Programmers often discover solutions while explaining a problem to someone else, even to people with no programming knowledge. Describing the code, and comparing to what it actually does, exposes inconsistencies. Explaining a subject also forces the programmer to look at it from new perspectives and can provide a deeper understanding.
the number of times I found the error, while describing the error on stackoverflow …
Is always funny when you wrote 3 paragraphs just to discard them all because you found the error while typing.