Mostly do it for non-fiction, not very nescisary for fiction and if anything could disrupt the flow of the story, with the exception of a reread where you may want to remember quotes you find compelling

Specifically in textbooks or art history books cause it's good for reference. I had a teacher who wrote a book and annotating it where I had questions was really helpful cause I could quickly find them and ask questions more directly as I can be quite rambling

Also worked well for philosophy stuff where I liked a particular quote or needed to double back pretty constantly to remember or reread things

I think it helps with my memory too, like books already have the advantage of accessing spacial memory and notes or underlines seems to dement it even further. like I remember stuff well reading a book but if I underline or specifically notate it I can specifically recall exactly where it was on a page. which let's you just leaf through the book very quickly and stare at the spot where you remember it to find it super quickly.

Most important thing is if everything is annotated noting is, that's why doing annotation on a second pass is usually the best move so they don't get to repetitive

Also I've found that for my comm class essay assignments with a chapter reading usually about 15 and online :( which makes sense cause it's usually from different books, not a purist just recall sooo much less from them) that highlighting is pretty much useless :/

Also if you annotate a borrowed book from a person or library your going to book hell, nicer than regular hell but still hell ya know :(

TLDR: for me it's useful for non fiction with specific bits you may want to recal for quotation and finding quickly and for a second pass on a fiction book

Don't annotate books that aren't yours,

Probably don't need to bother annotating digitaly

If everything is highlighted nothing is