I answered yes to the first three, because I find myself doing it for a variety of reasons at different times. The most common reason is to keep track of my place when someone calls me or walks in to my office. The next most common reason is related to the first reason, and it's a shame that I have to do so. When you use text highlighting to track your reading (line by line), it's a sign that the site you're reading…
Usually I highlight from near the middle of one line to near the middle of the next line. If near the end of reading line N I highlight from the middle of line N+1 to the middle of line N+2, I can use the one highlight three times to help me track back: first I track back to the first line above the highlight (N+1), second I track back to the highlighted line (N+2), third I track back to the first line under the highlight (N+3).
Similarly highlighting multiple lines at a time it's easy to subconsciously remember which line of the selection to read next (first, second, third, ...) without having to visually track back, as long as the selection isn't very long (much like how it's harder to keep track of where you are in a longer paragraph).