I ordered two sets of the optima V2 firing pin assembly got them today also. They included the O ring in each set. I should be ready to go for along time.
Im pretty sure its there to keep as much contamination as possible out of the threads, even tho some can still get in there from the pin hole. Also it helps to keep it from loosening during use, tho that may not be intentional.
btw, not a stupid Q.