They need to be added because in some cases the food just doesn't supply enough no matter what you give. I can't remember this example exactly, but it would take something like 40 medium sized apples with peeling for a dog to get enough of one certain nutrient per day. The amount of vitamins needed is something that researchers have come up with though and it may change someday...
You also have to be absolutely sure that the calcium/phosphorus ratio is balanced or it can lead to heart and blood problems. The best way to make sure everything is balanced is to talk to a nutritionist.