This is the eye that guides the secondary line from the purchase system & into the rear of the swiveling cam cleat.
Whilst very tough in time this does wear particularly when highly loaded & groves can develop.
These groves increase the friction in the system & so limit the amount of vang that can be applied.
(The wear also slightly changes the geometry of the lead angle & so the force holding the cam cleat swivel on one side or the other).