The most important thing to know about UV filters is that they add an extra layer of glass on top of the lens front element. This can be a huge benefit when you are shooting in a dusty, dirty environment or where you run the risk of bumping against something sharp and hard. They are used for protection rather than any dramatic effects; better the filter than the expensive lens.The UV filter can be used at all times with or without the combination of another filter.
Those who use film cameras vouch that UV filters are extremely effective in terms of removing haze and the bluish light cast that happens due to the presence of Ultra-Violet light. Since film is much more susceptible to ultra-violet light than digital sensors, this is the reason why a lot of DSLR users don't feel they need UV filters.