Marinades are flavorings that penetrate the meat in a liquid form. Generally containing acidic ingredients, for example, lemon juice or soy sauce, the acid acts as a tenderizer and slightly breaks down the structure of the meat.
As a result, marinades are used more often when grilling tougher cuts of meat that just cry out to be tenderized; whereas rubs, or even marinating for a short time only, are better for leaner cuts.
Having said that, it is very popular to marinate chicken in a salad dressing for the flavor it produces. That’s not something I personally do, but I know my brother in law does it regularly.