retaining ring

I have recently bought a kit containing the Apeks deluxe webbed one piece harness, while following the assembly instructions it tell you that you should install these little elastic retainer loops, however, I do not see why they would ever be needed or used as they are only slightly below the...

Plastic is okay. I lost my gold one today. Fell off on a dive. Noticed it gone when I got out of the water. Aqualung Legend, Legend LX, or Titan LX Looks like the ring on the far right in this picture: The one that goes on the outside of the diaphragm cover... Thank you.