I used to make notes in margins of most of my books, not just bibles. Similar to college notes, it served to jog my memory when I saw them again later. My notes either refuted some printed passage, or pointed out something worth remembering and repeating.
50-70 years later, those notes are interesting to read. I still have most of the books, but not all the same attitudes or scholarship.
I don’t think it’s limited to Christians and Bibles. ISTM it’s never been unusual to underline passages and otherwise mark up books that (a) you own, (b) are highly important to your beliefs, work, and interests, and (c) that you read and study on a daily basis or close to it.
it’s not limited to Christians and Bibles, but I think it’s newish in Christians and Bibles. If you were planning on owning one Bible your entire life, or leaving the family Bible to your family, or had a reverential attitude towards Bibles, you didn’t mark up the pages.