Consider them to be minimal type additional mufflers. Increasingly common on later model cars since noise regulations on new cars has gotten stricter. Resonators are used to dampen exhaust noise better with a moderate muffler. This is generally a much less system than trying to dampen the noise to the same level with just a muffler.
Nowadays you'll also see "resonators" on air intakes on cars. Generally between the air filter and the throttle body, they dampen the sounds of the engine drawing air in. About all the hyped "cold air intakes" sold accomplish only eliminating the factory resonator and making the car SOUND like it has more power. In many cases the OEM setups actually do a better job of drawing in fresh air. IE, from behind the headlight or the grill rather than just inside the engine compartment as most kits end up doing.