Square and rectangular filter systems solve problems that round, screw-on filters often cannot resolve. To begin with, one square or rectangular filter can be used with many different size lenses. As long as you have the appropriate size adapter rings, in most cases, one filter will serve the purpose.
Another benefit is if you are shooting a landscape where the horizon line is off-center and you want to balance the light intensity in both parts of the scene, i.e. bright sky, shaded foreground . Rectangular filters allow the transition to be blended into the scene, often imperceptibly. This is because they can be shifted up and down in the filter holder to fit the scene as desired. Some graduated rectangular filters are available with soft or hard transitions from dark to clear. The soft edge is recommended for wide-angle lenses and the hard edge is recommended for normal and longer lenses.