You don't need the resonator. However, a lot of people that remove them find that they do not like the noise. The resonator makes the exhaust note a little deeper and more aggressive sounding. Resonators are built to resonate at a certain frequency that helps to change certain sounds. Generally, especially on trucks. The sounds that they are meant to change are the higher pitched frequencies. So, while they don't actually make an exhaust quieter. It seems they do because of those higher pitched sounds that travel further are resonated into a deeper tone.