I personally have tried 100% raw diets, and it wasn't too good for me, mostly because it's difficult to be 100% raw and not-a-vegan, and veganism doesn't work for me. A lot of people eat meat raw as a part of the diet but I prefer my meats cooked. I ended up staying high raw and that, in my opinion, is probably the healthiest diet on earth. My diet is about 80% raw fruits, salads, fermented and sprouted foods, 10% cooked vegetables and greens, and 10% meat, fish and eggs.
I do this version of the diet which is basically a form of the Specific Carbohydrate Diet. I do it because I have really bad autoimmune issues due to leaky gut and gluten damage (Im celiac too). It works wonders for healing the holes in your gut. The longer I stay on it the less my symptoms are.
Anyways though, my belief about why a lot of people experience huge health benefits while 100% is that 1) this diet makes for an excellent short-term cleanse and 2) Just by the nature of the diet people end up giving up most glutens, processed soy and other foods, which will give almost anyone a health boost whether they are allergic to these things or not. Everyone is going to feel better without bread in their diet.
There is just no reason to be 100% raw, in my view. Why miss out on warm soups, spicy stir fry, steamed vegetables? I don't buy the logic that everything cooked is toxic somehow. The same raw foodists who say that are the ones who say it's wrong to use spices like black pepper or cayenne, and that is just too far for me.