It's called default mode thinking. Or the default mode network [1].
And I agree, not letting your mind do this from time to time results in higher stress and less ability to focus.
"All of humanity's problems stem from man's inability to sit quietly in a room alone." -Blaise Pascal
And mostly reduced creativity.
I'm addicted to reading, I take my kindle and phone everywhere, so will grab them when I'm walking, taking a shower, waiting in line, going to the restroom... Between my kindle and my phone, I read a lot more books than I ever did but I don't digest the information as much as I used to. I also don't make as much associations between what I read and things going on in my own life. So, in a way, despite reading a lot more, I don't think I benefit as much from it.
Now, I'm purposefully forcing myself not to reach to my kindle when taking a walk so that my mind can wander as much as I do.
At some point on William Gibson's now defunct micro blog*, he's about to embark on the book tour for Pattern Recognition (so circa 2003).
I'll butcher his insightful phrasing, but he remarks to the effect of
> I think I'm going to stop blogging. The act of sitting at a laptop and writing these posts seems incompatible with my life as it exists on a book tour. The only free moments available for it to occupy would be ones where I'm sitting, momentarily caught between two scheduled activities and staring off into space. I have a suspicion these moments are crucial for my soul. So, until we meet again.
The comingled ambiguousness and specificity of the observation stuck with me.
* https://web.archive.org/web/20070123212506/http://www.willia...