You're thinking of a surface gauge.   For stop motion, it's used to estimate how much a puppet has been or should be moved.  It's also good if you're looking to have precise movement.  You can easily use a gauge to measure out each movement of your puppet so that it stays consistent. Is it necessary? No.  With programs like Dragon Stop Motion, you can easily look back at the frames you've already shot.  Onion skinning also makes the surface gauge somewhat obsolete.