Speaking for myself, it's completely out of my control. I highlight randomly while reading, and the highlights aren't necessarily focused on the section I'm reading. And just because I highlighted something doesn't mean I will pause and look at it. Maybe I enjoy the colored aura of highlighted text flashing and changing in my periphery. I've never seen someone in person doing it, but I've read other accounts online. And this is definitely the most I've ever thought about it.

It's interesting to read other people highlighting to keep their place. I've never thought to do that. I normally just scroll the paragraph I'm on to the top of the browser.

I can't speak for everyone, but for myself and people I've discussed this with in person, it's not intentional; it's more of a tick.

That said, I find it does come in useful for keeping track.

I do it to mark where I'm at in the post if I need to switch to something else temporarily. Makes it easier to continue from where I left of.