This device is used to fix ball marks on the green. According to a Note in the Decision 20-1/16 of the Rules of Golf:
Note: The position of a ball to be lifted should be marked by placing a ball-marker, a small coin or other similar object immediately behind the ball. If the ball-marker interferes with the play, stance or stroke of another player, it should be placed one or more clubhead-lengths to one side.
This Note means that a small coin is recommended but is not required by the Rule. You can mark your ball with any object---a club, a tee, a divot tool, or even a red Mercedes 500 SL if you want to. The only provision is that you have to be able to return the ball to the same place on the green from which it was lifted.
