Why do people need diet pills for their dog?

I know that my vet actually prescribes diet pills for dogs. Now I fully understand why people take pills to curb their appetite because us humans have a problem saying no to food when it is constantly available to us. But dogs are different, we have to give the food to them in order for them to eat. All we have to do is restrict access to the food for the dog not to gain too much weight. Why do we need to give them a pill? Anyone have a thought on that?