What is the advantage of an air intake upgrade on a motorcycle?

To increase horsepower. However, adding a less restrictive air filter will require other modifications in late model motorcycles. You will also need to add a less restrictive muffler and the carburetors will need to be "re-jetted". By simply adding a less restrictive air filter and not doing the rest of these modification you may actually cause more harm than good. Your motorcycle was tuned at the factory using the components as is. When you change one component it affects the performance of the others.