The ferrule spreads load between the tip and the shaft's wood or composite. A crack, yellowing or play can reduce accuracy and expose the shaft end to moisture.
Materials
Manufacturers use polymers, metal and composites. Fit to shaft diameter, tip hardness and machining method matter more than a material name in an advert.
When to replace
Replace a ferrule when cracked, lifting at the edge or impossible to flatten without removing too much shaft. A technician should preserve coaxial alignment.
After service, check the face, tip bond and joint stability with gentle test shots.