Mouthwash is not considered essential in order to keep your teeth clean. Adequate brushing and regular flossing is usually more than enough.
However, depending on your circumstances, certain rinses may be prescribed by your dental professional. Such rinses tend to aid in gum problems such as gingivitis or gum disease, however they should only be used short term due to side effects such as staining.
Popular supermarket mouthwashes such as Listerine or Colgate provide minimal benefits but do help to provide that minty fresh feeling. If you are going to use such products, ensure they are alcohol free- this will limit the chances of dry mouth and reduce your risk of oral cancer.