WWE likes to own the intellectual property for their wrestlers' ring names, and they can't own someone's name, so they use different ring names for them. Plus, there's the obvious cases when someone does a gimmick, and many times in the indies guys use different names because they want a different name to define his character. Dean Ambrose's ring name was Jon Moxley in the indies, the name Jonathan Good would sound sort of weird for a sadistic heel gimmick (unless it's ironic), don't you think?