You're right, John. It's a combination of diet and exercise which is recommended by every sane physician whose work I've read.
BUT! I know at least a dozen people who swear that they can lose weight by finding the right diet and sticking to it. It's a good thing that they haven't put money on it.
Exercise also increases blood flow to the organs (including the brain), and blood with healthy amounts of oxygen in it (induced by exercise, unless you're into O2 doping ) refreshes and invigorates the organs.
I have a personal belief that blood flowing at a quicker rate through the body (such as caused by exercise) acts like water in a creek bed, washing away the trash sitting along the banks by way of HDL, the good cholesterol, acting as a sand blaster of sorts. At the very least, exercise stretches the arteries and veins, possibly keeping them from premature hardening.
The above is specuation, of course, but let me have my illusions. Thanks