I often use highlighting as a positional anchor, often when tired. My eyes begin to lose focus and I can end up reading the same line over-and-over again.

In some cases contrast highlighting doesn't work, particularly on macOS since it doesn't seem to change the text color on highlight. It's notoriously bad in my experience when trying to read a downvoted comment on HN. You end up with a super light text on a very pale blue, that is even harder to read.