When cooking a piece of meat, using marinades can really add flavors throughout the entire piece. Seasonings can only reach the surface of the meat and may not alter the flavor inside. A good steak marinade will infuse into all parts of the meat if given enough time to penetrate.
Marinades usually do two things: (1) they will season the food, and (2) they will tenderize the food. Different meats have different qualities; therefore, you need to consider more than one marinade. Marinades that are suitable for a tough cut of beef may not be suitable for a salmon filet.