Graduated Neutral Density (GND)
Also known as a split neutral-density filter, GNDs selectively transmit light. Therefore it is essentially an ND where only part of the filter is darker, which allows you to reduce the brightness in part of your image. As a result, it is particularly useful in a contrasty scene with a bright sky.
A Graduated Neutral Density filter helps balance the sky
While these filters do little to affect your image, their main purpose is to protect the front element of the lens from dust and scratches. That being said, compromising on the quality of a UV filter may degrade the quality of your images. The best reason to add a UV filter would be for lenses that need a filter in place to complete its weather sealing.