what I tell women who ask me as a midwife, is basically it teaches you how to centre yourself, breathe (sounds weird learning how to breathe), but how to get in a zone I guess.
the big thing I think is, yes the aim is a 'drug free natural birth', but it is also handy if things don't go to plan. If for some reason you require and emergency csection, or have something that wasn't in your initial birth plan, it can help you deal with it and cope with it better, help you calm yourself and remain in your special place and centre yourself and cope better when things go off the rail