why should we highlight in books, why should we write things in the margins? how do we highlight, what makes the brain go this is something i want to see again later

maybe when reading something for a test, the goal is clearer and so what needs to be retained in memory becomes easier to pinpoint. but in situations like reading for fun or a book club how do i highlight? i want to be able to go back to my highlights and leave notes, i want to understand my reading habits more and i want to make something help me get there!