|MadSci Network: Neuroscience|
I expect you could ask that one at a psychologists' convention and never get one answer that satisfied everyone.
However, I'm not a psychologist, so I'll give it a whack.
We all have preferred modes of thinking. I'm strong on computation and verbal, but poor on visual. So sometimes, the exercise of putting something into words helps me focus my thoughts.
If I had pen and paper, I could write it out. If I didn't, but I needed to get the thought into concrete form, I might begin talking.
The brain is extremely compartmentalized. It could be that one part of the brain vocalizes the thought, and another part hears it, providing an alternate route to the internal communications flow.
Neither of these answers is to be taken as definitive, of course. I'm just thinking out loud...
Try the links in the MadSci Library for more information on Neuroscience.