Dogs are domesticated from wolves, cats and tigers belong to one family. They ate the meat directly as food in the previous time. Just due to the evolving of the human domestication, they begin to eat the pellets. For the freeze dried products, they can be fed as main food and also as treats.  Just due to the cost of freeze dried pure meat is much higher, so, normally the peoples use them as treat to feed the pets. And gradually, there appear freeze dried mixture products as main food.