Resonator

Is the only purpose of an exhaust resonator (separate from a muffler) to 
quiet the sound of an engine's exhaust or are they used to shape the 
exhaust impulses to gain better breathing?

Would an engine normally be better off without a resonator (performance 
wise) assuming one didn't care much about the exhaust sound?

I think only the later Spridgets had a resonator. Why not the early ones?

Tim