How we taste is important because taste, not smell, gives you the nutritional information of foods. Carbohydrates and sugars are sweet; minerals are salty and bitter; and proteins, fats and vitamins have no taste (except for Vitamin C). Umami, however, a proposed, yet contentious new taste type, corresponds to a taste for glutamate, a chemical that is derived from the breakdown of proteins. Umami might explain why steaks aged for a long time are tastier than lesser-aged ones.
In general we tend to prefer sweet and salty to sour and bitter, a note that sends a signal to the brain that a food may contain acids or toxins.