The exhaust system on your car, especially the tailpipe itself, affects the car's overall performance and appearance more than you may realize. The entire exhaust system, not just the muffler itself, quiets the sound of the engine, pulls oxygen out of the pistons so the engine can maintain proper combustion, and also filters fumes and emissions so that less pollution is released into the air. While the exhaust system that was installed when the car was manufactured may be sufficient for the car's performance, note a few reasons to consider having a custom exhaust made for your car specifically, and why this can be especially good for larger vehicles or restored cars.
The exhaust system and tailpipe on your car may muffle some of the sound from the engine, but heavier engines especially may still be very loud. One reason for this is that many tailpipes are just bent or crimped to fit around other parts under the car body, and these bends can allow air to still rattle and vibrate as it passes through those pipes. A pipe that has been rounded, typically with a ring roll or mandrel bend, can allow air to pass through more smoothly; in turn, the engine is better muffled and will become quieter. If your vehicle has a heavier engine or an older engine that tends to knock and rattle while it's running, a custom exhaust can help to keep it quieter overall.
If you want to tow anything with your vehicle, you need more horsepower or torque. This is what allows an engine to resist drag while still pumping the pistons in order to create combustion. With a custom exhaust, and especially one that has been rolled and not just bent, as mentioned above, the vehicle will be able to remove more dirty air and fumes from the engine. In turn, the engine can bring in more clean air and create more power and more needed torque. If you've recently bought a caravan or boat or will now be hauling a trailer for any reason, consider adding a custom exhaust to your vehicle for the power it will need for towing.
If you've invested in a great paint job and other detailing for your car, why have an old and drab tailpipe poking out the back end? Dual exhaust pipes in a chrome or stainless steel can make your car look sporty and classic, which will be a great touch for a classic car you've restored.