Resonators are basically air baffels. The purpose of resonators, generally is to allow a certain acoustic sound to be generated.
For purpose here...A resonator is designed to give a Magnum, Charger, or 300C a more subtle and yet aggressive sound, which would be expected to hear as compared to the V6 verisons of the same vehicle.
Certain cars with big engines, for example luxury cars would be expected to be fast, but not loud, so a Bently sedan would have a resonator added ( to be more quite), as compared to a mustang which would be , by deisign, to have a more throaty sound.
Because resonators, also deflect interiorly the air flow, they can also use there design to increase the air flow exiting, as compared to a regular exhast which just lets the air spit out.
This in turn can alow a better combustion, and explusion process, which in turn saves gas by increasing the used fuel (same reason why air filers (like K&N, save more gas then a standard air filer). Some fuel may not be used in your engine.
Hard for me to explain, but to make it simple....An engine is designed to let a certain amount of air and fuel to mix. If the fuel (exploded by the spark plug), does not have extra air, some fuel may not burn. and is in effect expelled and wasted (ever smell an old cars exhaust???). While by allowing extra air (air filer), the fuel is allowed more combustion (more horsepower), and the expelled fuel (exhaust) is exited from the engine faster, allowing fast combustion...faster car...
Note: Combustion, is an explosion of heat and force. Most newer cars use the Alumine engines, because an alumine engine is more flexable over time then a cast iron block engine.
However, for racing etc...a 600 HP race car also has the engine broken down after every race and componets replaced etc, and by design can stand substantial pressures....so dont mix the race car, and the (lucky if I change) my oil every 3000 miles daily car you drive.