Mine has one too; it acts like a kind of spring to keep the whole thing tight and to stop any wobble in the blade when you screw it all together.
I don't think it's strictly necessary, but it's a good thing, and if your's has gone awol, I think it'd be worth asking Fiskars about a replacement.
I know I've dropped it a couple of times when I changed the blade; last time it ended up under a dresser in the kitchen and was a right pain to find.
Mary