The manufacturer states it best.
# The Royal Chuck Stop provides a simple, consistent method for locating short parts in a 3-jaw lathe chuck.
# Captive, web-shaped design eliminates the danger of using spacers or parallels that could come loose and be thrown from a spinning chuck. Three strong magnets further secure the stop to the chuck face.
# Stop allows short parts to project past the chuck jaws, providing clearance for facing operations.
# Setup time is greatly reduced - no need to bore soft jaws.
# The Royal Chuck Stop is very accurate - front and back locating surfaces are parallel within ± 0.0004 inch.