The air filter ensures that clean air enters the carburetor or throttle body, which is then injected into the cylinder in the correct proportion with gasoline. The filter also stops dirt which could otherwise damage the engine of your scooter.
It is wise to replace the air filter for the first time at 1500 km with a new filter, you also clean the house where the air filter element is located. After the first replacement, replace the air filter element every 2500km or every year.