most insurance companies require a 3- to 6- month supervised diet like the one you described (some surgeons do as well, but it seems like more often it's the insurance company). A lot of us are required to do this. I found it VERY helpful. It eases the transition between pre- and post-op life so your change in lifestyle after surgery doesn't seem as radical. In mine, I was supposed to limit myself to 2300 calories (I was eating 3000+ before), gradually increase my protein and decrease my carbs, wean myself off carbonation and caffeine, practice separating drinking from eating, exercising 3-5 times a week, etc. I lost 57 lbs in the process and I really think it helped with the transition.
as far as "losing weight with it and therefore not needing surgery" goes, I'd lost 50-60 lbs a gazillion times before - but the problem was, I could never keep it off. I'd hit a wall, bad habits would start to creep back in, I'd stop monitoring myself as closely, and the weight would gradually come back on. With surgery, I've been able to lose ALL my excess weight and keep it off. Plus when you do this on your own, you're constantly fighting biology. Your gut bacteria, hunger hormones, metabolism, etc, are all fighting to put that weight back on. Surgery resets all of that, so even though it's not easy, it's not an impossible battle, either.