Chain stoppers secure and hold the anchor chain whenever the windlass is not actually in use. They prevent damage to your windlass drive shaft and gears caused by excessive loading. Mounted in front of the windlass, they are engineered to withstand the heavy shock loads imposed by waves when riding at anchor, as well as the strain of setting or breaking out the anchor.
A chain stopper is an essential piece of deck hardware when retrieving a rope and chain rode using a capstan and chainwheel windlass without a chainpipe (like Maxwell's VW series of vertical windlasses) where, when you reach the rope/chain transition, you would use the chain stopper to hold the chain while removing the rope from the capstan and engaging the chain on the chainwheel.
When the anchor is stowed in its bow roller, a chain stopper should be one of the components of your securing system to prevent accidental anchor deployment while underway.
Fisheries recommends using a chain stopper or snubber in conjunction with all windlass systems utilizing a chain rode.
The single best investment you can make to protect your windlass from damage during use
• The lever is a convenient feature which helps you to lift the stopper gate during deployment - keeping your fingers safe
• Sized for use with particular sizes of chain
• The gate pin is removable for placing the chain in the stopper