A couple of reasons I can think of.

- Reading helps hone your ability to focus your attention on one thing for sustained periods.

- Getting back into the habit of reading by reading "trashy" or otherwise easy books can serve as a stepping stone to reading more difficult or rewarding books. This worked for me when I fell out of the reading habit for a while and was bouncing off books that I wanted to read that were less immediately engaging.

I would also say that while I agree that being alone with your mind is quite healthy and something we would all ideally make time for, I don't think that it's the alternative most people are opting for over reading.

I think it's better iff it's used as training for reading better books. Definitely not worth doing if you insist on only reading that forever, though.