Add color to part of a scene where color may be weak or absent. Can be also used for color effects which contribute to the cinematic drama of a scene.
All Graduated color filters are part clear and part color. They are designed to be used in professional matte boxes, so that they are readily positioned at the exact angle required to create desired effects.
Typically soft-edge filters with wider gradient transitions are used with short to medium focal length lenses because they blend in to most scenes better.
Hard-edge filters having narrow gradient transitions are preferred for use with longer focal length lenses or for straight and narrow transitions, like the sky above a flat ocean horizon.