In this video, if the resonator shut by finger the engine sound become noisy and when it released the engine sound become normal again...thus the Air Cut Valve and Resonator give something essential function then?
Other name for this is AIS. The primary function is to have cleaner or fully burn fuel emit from exhaust. It will have more air through the AIS to burn extra fuel.
Some modify bike need to remove this because it will introduce backfire.