A slim type of filter such as the B+W 77mm MRC Slim UV Filter offers lens protection to very wide angle lenses without causing vignetting on a full frame camera body. The filter also completes the weather sealing on certain lenses.
Why aren't all filters slim? Good question - there are several reasons ...
First, a normal lens cap does not mount on the end of the B+W 77mm MRC Slim UV Filter. B+W supplies a snap-on/push-on/slip-on friction-fit lens cap to protect the filter. This works, but the cap tends to slip-off much more easily than the standard lens caps.
The B+W 77mm MRC Slim UV Filter does not have front threads (which allows it to be "Slim"). While mounting two filters on top of each other is not usually recommended, being able to add a Circular Polarizer Filter to an already-mounted UV filter is helpful in harsh conditions (such as salt-water spray).
The cost of a slim filter is higher than a normal thickness filter - something I'm sure all of us care about.
According to B+W/Schneider Optic, a slim filter is required (to prevent vignetting) when the effective focal length (field of view crop factor included in calculation) of a lens is wider than 28mm. However, I am finding this to be somewhat lens dependant. Stacking filters will create the need for a slim outer filter at an even higher focal length.
On a digital SLR camera, the primary purpose of the B+W 77mm MRC Slim UV Filter is lens protection for expensive lenses (such as the Canon L Series Lenses).
I'll direct you to my B+W UV Filter Review more more information on selecting a UV filter.