Sorry if this is a dumb question , but what exactly does the little black cylinder do? does it serve any purpose ? ive seen aftermarket Y-pipes without them , just curious.

 

That is what Nissan calls a "Insulator, Exhaust/ Dynamic Damper." It is "supposed" to reduce NVH (Noise, Vibration, Harshness) within the exhaust piping. Whether they are effective or not is debatable.

 

What this does is add mass onto the exhaust system, which changes it's normal resonant frequency. The more mass, the lower the frequency. The idea is to avoid having multiple bits of the car have the same resonant frequency, so that you don't get rattling or buzzing.

The VQ engine is not a smooth engine, so all the rubber mounts and harmonic dampeners are all there to try and break up the powertrain vibrations so they are less noticeable, which is cheaper than fixing the engine harmonics itself.