Can someone tell me if I'm wrong in the following or if I'm over-simplifying things:
Protein is used to build tissue such as muscle, organs etc. You need protein for this. Fat won't do it, carbohydrates won't do it. If consumed in excess, it converts to glycogen which is then stored in muscles and liver, and in a poor diet, as fat.
You need fat in order to help with vitamin absorption, hormones and a means of energy. Again, in excess, stored as fat.
You want carbohydrates because they have vitamins, minerals, fiber. They are used first for fuel thus allowing your body to use protein for muscle repair. They are stored in muscles and liver for later use. Consumed in excess, they are stored as fat.
Protein and fat are a must. Carbohydrates are not, but they are beneficial and shouldn't be avoided (With the exception of the junk food carbs).
So, if you're looking to lose weight, you want to make sure you have enough protein to prevent muscle loss, carbohydrates to prevent the use of protein for energy, fat to feel full and also to be used for energy.
Is there anything you would add? Or did I make no sense at all here?