In bright sunny conditions, UV light can add a blue/purple tinge to your pictures. It;s most obvious when shooting in show , at high altitudes or in sunny climes. Using a UV filter in these situations will get rid of this colour cast and may also boost image sharpness slightly by reducing the misty effect on distant subjects.
However , that is not the primary reason for purchasing an UV filter . More important is the fact that a UV is perfect for protecting the front of your lenses from scratches , dust ,water and impact. Replacing a filter is far cheaper than replacing an expensive lens. As UV filters are clear and colourless they have very minimal effect on your images and no effect on the exposure. Skylight filters do the same thing , the only difference being that a skylight 1B adds a slight warm tone to shots , whereas a 1A is completely clear. So when you buy a lens make sure you buy a UV filter as protection by screwing it onto the front of the lens.