Makes me wonder.
Agile development must be hard. Why else would there be so many Agile coaches? When people were doing waterfall, I don't think there were that many "Waterfall Coaches". Think about it. How many of your contacts in the software world were "Waterfall Coaches"? None of my personal contacts were coaching waterfall. On the other hand, I know of at least half a dozen who are in this business - either fulltime or part time. And I'm running into many strangers who are Agile coaches. Or at least want to become one.
For a methodology that is supposed to be simple, why are there so many coaches? Perhaps it is not simple. Agile is supposed to simplify but that's not the same thing as being simple. Perhaps the highly adaptive nature of agile methodologies necessitates a coach.
Essentially - what does an agile coach do? If you are a team that's already practicing an agile methodology, she'll probably take a look at your methods and suggest improvements. Should I use the word prescribe changes? Funny. We choose a methodology that's not prescriptive for its ostensible benefits and then we hire a coach to prescribe methods.