Sweat is important. Sweating prevents heat exhaustion and heat stroke, which happen when the body overheats. Sweat on the skin cools down the body as it evaporates and helps keep it at its normal temperature of 98.6 degrees.
Converting a liquid to a vapor requires heat energy, so evaporating sweat literally pulls heat away from the body. Around one liter (or one quart) of blood can drop about one degree Fahrenheit from just one small bead of sweat.