As arthritis progresses, X-rays may reveal bone spurs (osteophytes) and cavities (cysts), indicating that the disease process is becoming advanced.
The way that cysts and spurs come about is that the disordered bone tries to desperately heal the damage in its joint surface. Bone is dissolved (resorbed) from some areas and built up in others, but in a disordered way, leaving holes in some places and heaped up areas in others - cysts and spurs. The cysts and spurs contribute to joint deformity and can cause problems in their own right.
Bone spurs may not be symptomatic, but often they are tender when the skin is rubbed over them, and they may cause irritation of other tissues in contact with the spur.
Cysts and spurs can be easily seen on X-ray and give the surgeon a clue that the joint destruction is becoming advanced.
Generally that is their real significance, but sometimes surgical removal of a symptomatic spur is warranted to provide relief from local symptoms.