Here is what the registered dental hygienists do on a daily basis:
They remove extrinsic stain and discoloration on your teeth so that you can maintain a youthful smile;
They remove the hard deposits (calculus) in your mouth, including the hard-to-reach areas, keeping the gums and the hard tissues (bone support) healthy;
They prevent and monitor the progression of gum disease, which at an advanced stage can lead to tooth loss or other serious health problems;
The dental hygienists will teach you effective ways to keep your teeth clean and healthy. They may also recommend fluoride treatment to individuals that are cavity prone.
They can check and monitor the conditions of your dental problems (gum recession, how loose some teeth are, etc).
How often you go for a dental cleaning or check up depends on many factors: how diligent and how effective you are about brushing and flossing, how fast calculus builds up in your mouth, whether you smoke, whether you have underlying medical issues or you are on medications that cause dry mouth or gums to overgrow. According to the Canadian Dental Association, most people need a cleaning every six months. Your dentist may recommend that you visit more or less frequently depending on your needs.