Well basically due to emission and noise regulations abroad these bikes come with a catalytic converter and muffler which reduces the noise the bike produces. However this also ends up making the bike lose some power as the exhaust is not free flow.
So basically what this y-pipe does is that it eliminates the cat and makes the exhaust gases free flow. This leads to an increase in horsepower (anywhere from 4-8 depending on the system) and also improves the sound of the bike. The cat also weighs around 7-8 kg's so the weight loss also helps improve performance. I could've made it even louder by eliminating the honeycomb in the headers, but i left it in as it would've made it too loud.
This is the CAT:
The headers with the CAT removed & ready for Y-pipe fitting:
