It's communication! And the first step in communication is communicating with ourselves.

Step 1: In your head Step 2: Aloud (to yourself or others) Step 3: Written

Ever notice how that thought you have in your head that makes sense suddenly is incomplete/non-sense/wrong when you try to verbalize it? Our minds are great at tricking themselves into believing they understand things.

Writing is the next level. Take something you talk about and try to coherently communicate it on paper. More incompleteness/non-sense/wrongness may be revealed.

For something you are an old hand at, you may not notice much difference between what's in your head, what you can verbalize and/or write. But take something nascent and the contrast will be sharp.

Verbalizing and writing are two tools we have for communicating first with ourselves and then with others.

I never really thought about it, but when I write, I do tend to talk to myself. I also often write out concepts that I have difficulty verbally expressing. Interesting...

Not a study, but...in Ron Chernow's bio of Alexander Hamilton there are a couple of places where he mentions that AH was notable for talking to himself while walking from place to place. [1] Your comment made me think of it because his tremendous impact and voluminous output made that behavior seem odd. But, perhapse they where related.

[1] https://books.google.com/books?id=4z5eL5SGjEoC&pg=PT438&lpg=...

Maybe go for the negative? I can't cite them off the top of my head, but I'm pretty sure over the years I saw different headlines and titles about studies where it was shown that language has a strong impact on thought. More precisely what I recall is that people who don't learn a language have "development issues"[0]. As this isn't my area of research, I'm quite bad at remembering most about it, other than the anecdotic.

[0] I'm not sure this is the correct (much less the politically correct) terminology.