Why do animals need the smell sense?

Well, it depends on how you define the terms "taste" and "smell." The way most people use "taste," to mean the sensations experienced when eating a food or drinking a beverage, smell is definitely a large portion of the experience. If, however, you define the terms "taste" and "smell" more scientifically, so that taste refers to the sensations elicited by stimulation of the taste receptor cells and smell refers to the stimulation of olfactory neuron, then these are independent senses, both of which play an important role in the experiencing of foods and beverages. A more scientifically accurate statement would be that "Most of flavor is smell," where flavor is defined as the combination of sensations experienced when eating or drinking, especially, taste, smell, and chemesthesis.