I don't know why they exist. I thought the whole point of a marker was to remove the obstacle of a ball potentially being struck, and replace it with something flat that a ball could easily roll over with deviating much. You can't roll it over a poker chip, it would hit it and change direction - so it's not fit for purpose.
If the marker is on the line of another player's putt then you can move it.
There is no need for a marker to be on the line of a putt and the ball to roll over it.
You can use anything to mark the position as long as you're able to replace the ball in the same spot.
Brian Barnes used a beer bottle..