These filters are used by many photographers to protect the front element of their lens from abrasions, dust, and moisture. They also have a very subtle warming effect, which can be useful to counter the "cool" colors you sometimes get in bright sun or in shade. They're available in sizes to fit most EF lenses.
Reviewed by 1 customers
I bought the filter to protect a new 16-35 mm lens. The filter arrived by mail dirty. It had a film of dirt that had to be cleaned prior to use. I have since removed the filter and replaced it with the more expensive and better filter from a competitor store.