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.
Works with writing fiction, too. I read the section aloud and explain the reasoning behind the choices, and that helps me figure out what isn’t quite working. Also, reading aloud makes awkward dialog stand out more so than reading silently.