Evolution of the species. On the humorous side: They tried being free weights, but it didn't work out. More on the development and domestication of dogs: Our dogs descended/evolved from the Grey...
Dogs are creatures that are there to keep us company. They are like a friend but a dog will (mainly) go on walks with you, play frisby (or ball) or just sit quietly and listen to you (some do bark)....
in general, there could be two realistic answers to this.
The first as a joke ... none, dogs don't have feet they have paws.
The second answer forgiving the feet/paw issue and presuming you meant...