On either end of the driveshafts on most cars today, there are C.V. gaiters (”boots”) which keep the special grease in the C.V Joint and the dirt and grit out.
(* “C.V. Joint” : Constant Velocity Joint)

These are made of neoprene in the main.
When the c.v. gaiters split, (as shown in the above photo ) as they would do with general wear and tear, the grease runs out, leaving the c.v. joint vulnerable to becoming dry and the bearings inside the joint breaking up.
To prevent this expensive replacement, it is less expensive in the long term to replace c.v. gaiters and re-pack with fresh grease before the joints run dry.