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.
While I havent explained my code to a rubber duck I did find The solution to my Problem just because I started to write it down in order to get help in a forum.