Often, you want to balance light intensity between two areas within a scene. This is important outdoors to allow more sky detail while properly exposing the foreground.
Exposing for the foreground will produce a washed-out, over-exposed sky. Exposing for the sky will leave the foreground dark, underexposed.
Color-Grad Neutral Density filters are part clear, part neutral density, with a smoothly graded transition between. A Color-Grad ND.9-to-clear is often best for balancing sky to foreground.
Neutral gray appearance drops exposure.