Engines burn fuel and need to expel the combustion materials in the most efficient manner possible. The exhaust is the route these take from the engine to the back of the car but there are many considerations that need to be taken into account to ensure this happens in the best way possible. It is essential to get the flow rate just right, too low a flow rate and the power band will suffer.
You should view the head in your engine as a busy and complex highway for air and fuel. All the air that enters your engine has to flows through the head, mix with fuel and then it glows into and into the cylinder via the inlet valves. This air is usually moving into an at high speed and it can be dramatically hampered if it encounters turbulence or obstructions.
The basic job of the silencer in an exhaust is to minimise the sound waves coming from the engine which is basically a series of very loud explosions. In some countries people associate noise with power, probably due to the fact that older cars had inefficient silencers. In countries where cars have relatively recently been introduced like Japan the silencer was more efficinet and the generally desired exhaust note is very quiet and more of a buzz, so this is reflected in aftermarket parts from different countries.