Shelf Markers

I am currently student teaching at an elementary school. My cooperating teacher keeps telling kids to use shelf markers. These are paint mixing sticks that have been decorated with fun designs. I could not figure out what these shelf markers were used for, but I just did and I think they are amazing. The students look on the shelves for books and when they take out a book they put the marker in that spot. That way if the student decides not to take that book they will know where it came from and put it back in the correct spot. Anyone have other methods for students putting books back on the shelf?