Any time a food is brought to a temperature above 117 degrees F the molecular structure is changed the enzymes are destroyed. Unlike you, your pet does not have the ability to create these digestive enzymes so he/she must get them from the food they eat.
In addition most food's nutritional value is reduced or depleted when it is cooked. Below are the results of a 2003 study from the USDA which tested the effects of cooking on nutrient levels in a variety of food. The overall finding was that any heat exposure will break down and reduce the amount of nutrients available in the food. The higher the heat or longer the exposure to the food, the greater the loss. Each number represents the overall nutrient loss of food being "baked" which is basically how dry kibble is produced. Keep in mind that canned food is exposed to more heat and will high greater nutrient losses.