I have had the same thought many times. I find it annoying that part of my mind insists on sub-vocalising the words when I know what it is I am thinking already. But as others have said sometimes it is useful as you re-analyse the words once they are formed. Yeah, but most of the time it's an annoying waste of time.

It's like there is a big mess of interconnected thoughts, intuitions, sensations clustering into a thought, fumbling along nicely on it's own... and then the apparatus in my mind responsible for flattening it all into words jumps in, totally unrequested