They are used primarily for landscapes. First render your scene normally & then render the distance render. This makes a grayscale render with closer terraine darker. Then open your normal scene in Photoshop (or similar app with layers), create a new layer and fill it with a color that you want to be your "haze" color. Then add a layer mask & paste the distance render into it. It will diminish the "haze" color from the foreground to backround according to the levels of gray. Havn't tried the "blur" thing from fore to back however.