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 a programmer but at my last job I couldnt count how many times I would walk into our engineers office and start explaining a problem only to stop mid sentence and say “wait, I’m an idiot” then walk back out. Sometimes talking about it is all you need to make the pieces fit.