It helps me function mentally.
I think that it creates an alternative cognitive input besides just THINKING what you want to do next; now you are also HEARING what you want to do next. Some people have to make a to-do list for everything they will do - that's an example of SEEING what you want to do next.
I've always had difficulty with working memory, so maybe hearing myself say things strengthens the memory trace (or something).
An example is when I play my hidden object game. I go fastest if I read 3 items off the list and search for all 3 before starting on another 3 items. I often say them out loud, or whisper them to myself if someone else is around and I don't want to bother them.
When I used to work in an office, I would sometimes say things like "this is a good time to do that filing" or "now return phone calls." Since there were always other people around, I would say it sotto voce or basically, whispering. But I had to actually make the shape of the words with my mouth. It sort of propelled me on to my next activity.
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A finger in every pie.