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.
Not quite the same, but sometimes when I’m doing a task I like to talk through what I’m doing out loud as if I was showing someone else how to do it.