Without any clear coat protection, the ink will be directly exposed to airborne abrasion - and will eventually be worn down to nothing. Raindrops carry air pollutants. When raindrops dry on the surface of a billboard, the pollutants are concentrated - and can directly react with unprotected ink. Just as important as weather protection, is the fact that clear coat greatly enhances the appearance of a billboard. After all, the dollar value of a board is based on how it looks.
Clear coat makes any outdoor advertising look significantly better.
Without clear coat, a printed surface is not truly smooth. It has minute hills and valleys that scatter light at different angles. The hills and valleys result from the different thicknesses of ink - and the surface texture of the vinyl. When you fill in the valleys with clear coat, the colors become more vivid , because the light scattering is reduced. To prove this to yourself, just compare the same image with and without clear coat.