Often it is necessary or desirable to balance the light intensity in one part of a scene with another.
This is especially true in situations where you don't have total light control, as in bright contrasty landscapes.
Sometimes it is also desireable to add colour to part of the photograph.
For example, to add blue to only the sky.
Lee colour graduated filters are half-colour, half-clear, with a graduated transition for blending into the scene.