We call these gap fillers. The Rattle Rings nylon washers are only needed if you have a gap to fill in the middle of the shackle between the hook and your mounting bracket. The nylon washers are not to be used on the outside of the shackle between the cap screw and the shackle hook.
Please note: It is important that the cap screws are threaded all the way down and tightened onto the pin so that the threads on the cap screw reach the tapered threads on the pin to lock the cap screw in place. Using a nylon washer that is too thick will not all these cap screw threads to reach or lock down properly. Be sure to use the proper size washer to only fill the gap and nothing more.
For the 3/4 inch Jowl shackle the space in between the shackle hooks is 1.25 inches. Most bumper shackle mounts are narrower than this so the rattle rings are used in the middle between the hook and the mounting brackt to fill those gaps to reduce noise and give your shackle a more secure fit. This also reduced pressure from the edges of your bumper mount onto the pin to increase the life of your shackles.
Most 2.0 inch shackle hitch receivers have a shackle mounting width of 1.25 specifically for a 3/4 shackle so the Rattle Rings nylon washers are not needed for mounting your Jowl Split Shackle if this is the case. If your shackle hitch recevier has a mounting width narrower than 1.25 then the Rattle Rings may be needed to reduce noise and give your split shackle a more stable fit.
If you need more help check out this great install video that will help you through it...