Obese patients are at higher risk of developing chronic diseases (heart disease, diabetes, cancer...). So if we find a cure to obesity, we considerably reduce the incidence of these chronic diseases.
Hormones play an important role in our body. They direct the energy to our cells wherever it is needed. For example, growth hormones direct the energy from food, to make a child grow (his bones, his organs...). Adrenalin is needed when we have to run from the lion... (or from our boss), it helps the energy to go to our muscles for fuel. Insulin is the only hormone which tells the body to store fat. This is well known since the 1960s and taught in medical schools. Doctors actually make very skinny people gain weight by injecting them with insulin.
High levels of insulin drive fat deposit. Insulin prevents fat loss. Period.
Another proof is the enlargement of fat cells (called ‘lipohypertrophy’) in diabetics who inject insulin in the same place for years or decades. This is a picture of a 55 year old diabetic man who received insulin shots in his belly for 30 years.
A case of lipohypertrophy
Obviously it is not the absence of physical exercise which made him fat, it is the constantly high levels of insulin in his body.
This same thing is happening to us. What makes our insulin level high is carbohydrate in our diet (bread, pasta, sugar, beer = liquid bread etc...). If we have chronically elevated insulin levels, we will not be able to loose weight, not matter how much we exercise. Once insulin levels go down by cutting carbs from our diet, we loose weight even without exercise. (I am not against exercise. Exercise is very important for good health, but it is not the way to loose extra kilos.)
High carbohydrate in our diet -> high insulin -> store fat
This is not happening on a High Fat - Low Carb diet. Fat is the only nutrient which does not have an effect on insulin. In the absence of insulin the energy cannot be stored as fat in our bodies.
For more details, here is an interesting article on how we become fat.