I'm definitely a fan of highlighting (as I think you may have noticed).

I started highlighting a lot when there was the option on Medium to see what you and others had marked up after the fact. Who knows where that has gone. I often thought "I'll highlight that and write about it/refer to this piece or author later".

Also, I like to highlight things I just think are either an important statement or beautifully worded. I hope doing this both encourages the author (and assures them that people are reading carefully) and brings other readers' eyes to particularly good parts.

Of course some essays and poems are so good that I just want to highlight the whole thing!