Mouth rinses can have therapeutic benefits; others are only cosmetic in nature. Some have both attributes. The Food and Drug Administration even classifies mouth rinses this way.
In general, some therapeutic rinses with fluoride have been shown to actually fight cavities, plaque and gingivitis formation.
On the other hand, cosmetic rinses merely treat breath odor, reduce bacteria and/or remove food particles in the mouth. They do nothing to treat periodontal disease or prevent gingivitis.
In any case, make sure and look for indication on the label of a mouth rinse that is accepted by the American Dental Association.
Surgery or oral disease sometimes leads to complications for which a good quality therapeutic rinse is indicated. Even people who have difficulty brushing (because of physical difficulties such as arthritis) can benefit from a good therapeutic mouth rinse.
Caution: Even rinses that are indicated to treat plaque or cavities are only moderately effective. In fact, regular rinsing with water and use of good quality fluoride toothpaste are actually just as or more effective. Some rinses have even been known to causes severe irritation of the soft tissues in the mouth and other problems such as fluoride toxicity, discoloration or overly-sensitive teeth and gums (due in part to high fluoride and alcohol content).