Exhaust systems are an enormously popular upgrade for every type of motorcycle out there. I've owned and ridden dirt bikes, dual sports, track-only race bikes, touring bikes, cruisers, scooters—just about anything with two wheels, and people buy and use aftermarket exhausts on all of them. The most popular exhaust application for any segment of motorcycle cycle riding is the slip on exhaust.
A 'slip-on' basically consists of a muffler or silencer with a connecting pipe that fits into the rest of the stock exhaust. This replaces the end of the exhaust pipe where the exhaust exits the motorcycle’s engine. There are a few reasons why this is such a popular upgrade. First, it's easy to do. All that's required to install a slip-on is to unbolt the stock muffler/silencer and bolt on the new one. All exhaust manufacturers make model specific slip-ons in a variety of styles for all different types of motorcycles. They essentially slip onto a stock exhaust pipe.
Another reason a slip on exhaust is so popular is because of its visibility. The muffler is typically right out there in plain sight for the world to see. Often times, the stock muffler is much larger and usually much uglier than its aftermarket replacement. You can save a lot of weight if you're focused on performance, but the bike night guy is just as pleased with his new carbon fiber, aluminum, or titanium pipe on his bike for everyone to ooh and aw over.
Sound is a major reason for the upgrade as well. A lot of riders will describe their motorcycle as having a sewing machine sound when they buy it and that they would prefer a deeper growling sound. Many mufflers use different baffling from the stock exhaust or even none at all. That's why it's also important to know what the law is for your state or region so that you are not making an illegal upgrade to your motorcycle that will get you in trouble later by law enforcement.