I’ll never forget seeing a girl in a cafe highlight, line by line, nearly an entire page of a textbook. Watching the page curl from the wetness, almost as if writhing in pain.
I think it's a combination of a ritual and some sort of somewhat-more-elevated engagement (or wishing thereof) with the material. Somewhere between meditative ritual and attention aid. I doubt it helped much, if at all - except for Stabilo; they were the true winners.
The unfortunate part is that you can't then unhighlight a passage.
And it's not like some ADHD thing which is new. E.g. Stanisław Lem as a kid: "But because the act of reading was too passive, too simple, I would rest a chair leg on my calf or knee or foot and with little movements keep the chair at the very brink of falling."