Did you ever take something apart, then put it back together only to find a part left over? You may go through the steps to find out where it belongs, or you may choose to ignore it. 
 If you choose the second option, you may be in for an eventual surprise when the assembly either breaks down or falls apart for want of that missed item. RETAINING RINGS - WHAT KEEPS IT ALL TOGETHER

The constant section ring (in yellow) shown with this CV joint is designed to fit into a groove located inside the bearing race and also ‘couple’ with the spline shaft that fits into the center hole. Some people think it is okay to eliminate the rings, but many others disagree as to the consequences.

Constant Velocity (CV) joints that use constant section rings sometimes fall into the category of a retaining ring with a less-than-obvious purpose. CV joints are flexible joints that transfer power from the transaxle to the wheels, moving up and down in response to the road surface. Some people have claimed that when rebuilding CV joints using a kit for that purpose, it may be fine to discard the rings. This school of thought claims that once the shaft is re-assembled, there is little room for movement as it is virtually locked into place between the wheel and the transmission.

The photo to the right shows one end of an ATV CV joint that fits directly into its transmission. The retaining ring is designed to snap into a groove located just below the rim of the bearing race. A mating splined shaft is inserted into the race with a corresponding groove that also accepts the portion of the same ring protruding from the race groove. This coupling of the shaft and the bearing race is to secure positioning over any serious retention.

But service manuals are quick to point out the necessity of using rings as specified. Some purists believe that the hitting a pothole or even just the natural movement of the suspension system may actually cause the shaft to move enough to either break away from the transmission or damage the splines on the mating shafts.

So the wise advice to follow seems to be to make sure you install the rings that come with that CV rebuilding kit. Having one or more leftover can result in an invitation to disaster.