Clouds and precipitation are formed by the upward motion of air. Therefore, there must be a mechanism present to lift the air.
Fronts often serve as such a mechanism.
Air on one side of the front typically blows in a different direction from the wind on the other side, causing the air to converge, or pile up right along the frontal surface. Since this air has to go somewhere, it rises. As air rises, the moisture in the rising air cools, condenses and forms clouds and precipitation . For example, a cold front lifts warm moist air ahead of it as it advances. As the cloud droplets grow in size, they begin to fall back to the earth as precipitation.