I think Accident Assistance is only relevant to accidents rather than breakdowns. If you have an accident they will take the car away and agree to do the repairs through your insurance, it's a way of getting work for car repair firms it's probably better to do everything through your insurance anyway.
The breakdown assistance will help if you get a puncture, have a mechanical fault etc.. I'm not confident exactly how this works as they've changed it for the Mk 4 Jazz and their web site is as clear as mud about it, I started a thread recently asking if anyone had used the service but got no response so the supplied phone number hasn't been put to the test as far as I'm aware.