The original purpose of UV filters was to filter out ultra violet light without affecting exposure when used with film cameras, and to some extent remove the haze you get in your shots on very bright days. Digital sensors are nowhere near as sensitive to UV light as film, so there isn't really a problem there to start with nowadays unless using film Even then any half decent lens will be expensively multi coated and one of those coatings wil be a UV coating, so a UV filter these days is largely redundant any way you look at it.
Some people do tend to use them though for protection for the front element of the lens. I prefer not to, but in a very wet, dusty or otherwise hostile environment, then it undeniably makes sense.
EDIT. Canon do say that even 'L' weather sealed lenses are not completely sealed unless a filter is used to stop water getting past the front lens element. Exceptions to this are the very large super teles - you just can't get filters in those sizes, which is why these lenses rely on flower pot sized lens hoods for rain protection.
