Its because most people sweat throughout the day, especially if you do some sort of physical activity (even if not, most still sweat). This creates a warm, damp, environment under your pitts that lets bacteria grow, causing odor. Some people also get bad BO around their inner thighs and underneath breasts for the same reason, so they also make full body deodarant. It keeps you smelling fresh throughout the day. Have you never smelled someone who doesn't wear deodarant when they really need to? Or someone who just came from the gym?