A UV filter cuts down the amount of UV light that reaches the film. You cannot see UV light, but films do - some films more than others.
UV light tends to bounce around amongst the dust particles in our atmosphere (aka haze) so film sees more of the haze than our eyes do. A UV filter reduces that effect.
Try shots with and without the filter, on hazy days and days where the air is cleaner (like after a rain storm). You may very well see clear differences.