Gesticulation

Why do people gesticulate when talking on the phone? It's not like anyone can see them doing it? Who does it benefit? To me it seems like a very unusual habit. Which is why I can't understand why I seem unable to stop myself from doing it.Even as I type, I will at times pause and point at the screen with one finger to sort of get my baring and make sure I know what's going on. If I don't do this,