ORIGINAL: mot250
That, my freind is for the noise police. That intake expansion tank helps keep air intake noise down, allowing the incoming air to expand, contract in that tank rather than pulsate with the actual intake air needs. I'd consider cutting that off completely and sealing the hole but I will be adding a CAI kit soon enough as you will be.
Thats it, just to keep noise down. It does not really hurt anything if you keep the stock air intake on. But these engines need to breath so install a CAI.