I'm not 100% sure which part you're referring to, but if it is the part that attaches the front of the fire engine to the rest of the vehicle, I believe it is meant to allow weapons to be placed in the hand without that piece getting in the way. It's not totally necessary for Energon Optimus himself, but for other compatible toys such as the Energon Saber or even the Requiem Blaster it might be very useful. Alternately, it might just be a vestigial design feature.