It stiffens up the front of the frame rails as they are sticking out unsupported quite a bit. The ARB stiffens things up enough that you don't really need it, but with the stock bumper on there you may get a lot of movement of the bumper; possibly to the extent that it could bump on the fender edges and such on rough roads. I don't know that for sure, but I do know it stiffens things up; even with the ARB on. It may also help to some extent in the event of an accident by helping to keep the frame rails together.
Anyway, I would definately reinstall it. It's a pretty big piece of metal so it shouldn't be THAT hard to find!
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ChuckH
"Every morning when I wake up I know it's not going to get any better 'til I go back to sleep again!" Al Bundy