On a digital SLR camera, the primary purpose of the B+W 77mm MRC UV Filter is lens protection for expensive lenses (such as the Canon L Series Lenses). I regard the B+W MRC UV filter as the best UV filter available.
There is an ongoing debate over the use of UV filters for lens protection. Neither side is wrong - they simply have differing opinions - and the freedom of choice.
The non-UV filter users say that their lens hoods provide all of the protection they need. They say UV filters degrade their pictures. Their contingency is that they can have a damaged lens repaired under their insurance coverage (check with your insurance agent for your specific coverage details).
Of course, an insurance claim does not put the lens back into service immediately. And an insurance claim puts a record with your name on it into the insurance company's shared claims database. Too many records in this database in a short time period will cause you to be considered a high risk.
I am a member of the UV filter users. A high quality UV filter such as the B+W 77mm MRC UV Filter will not degrade the final picture in most instances. A very bright light in the background of your picture *may* cause the filter to generate a small amount of flare, but I rarely detect any other filter-caused image defects. If I scratch the filter, I can simply remove it and keep shooting. And yes, this has happened to me (using a $1,000+ lens). I feel much more comfortable cleaning dirt off of an $80 filter than I do from a very expensive lens. I feel a little less protective of my lenses when they are protected with UV filters as well (I might let a friend actually hold it!). Note: A UV filter is not going to save the lens from a significant impact. You will still want insurance coverage if the lens is valuable.
Even non-regular filter users will want a filter when shooting in very dusty or harsh conditions (think salt water spray). Some lenses require UV filters to complete their weather sealing.
Some of the Canon Super Telephoto lenses cannot take screw-in UV filters (such as the Canon EF 300mm f/2.8 L IS USM Lens and the Canon EF 500mm f/4 L IS USM Lens) but have a front element designed for protection exclusively. Is this Canon's endorsement for protective filters?
I personally do NOT consider a low end/low cost filter to be acceptable. Many of these exist - do NOT buy them. They will degrade the quality of your pictures with contrast and flare being the primary issues. Image sharpness can be affected. You spent a lot of money for your camera and lens in quest of good quality pictures. Don't throw away your investment with a cheap UV filter.
B+W and Heliopan make excellent filters. Although many brands of UV filters exist, the quality and performance of B+W MRC and Heliopan SH-PMC filters place them among the world's finest. Black anodized brass rings reduce binding or cross threading and to ensure optimal alignment. The MRC and SH-PMC multicoating prevents light from being reflected off the surface of the filter - you want the light to go into the lens. High end Hoya filters are also good, but can be a pain to clean (they tend to smear).
I'll leave you with one last tip - If you are going to use any lens filter, be sure to know how to remove a stuck lens filter.
I use B+W MRC UV filters on most of my lenses.