Being talked down to is very irritating to most people, but sometimes your project just calls for a condescending voice. If you need the perfect holier-than-thou, patronizing tone, you're in the right spot.
Some voices just make you grit your teeth and resort to all the inner peace techniques you've learned in your life. The condescending voice is perfect for when you need that unbearable, ivory-tower feeling in your projects. Not every character is meant to be loved by the audience, and sometimes you just need a foil for all the good guys in your ensemble; or maybe you've written a part that's more snark than person, and need a talented read that will truly make that quality shine.
There are times when a character is so snarky so snotty and superior that they just stick in people's minds. A condescending voice is perfect for those characters you love to hate, and even more when they get their due comeuppance and are knocked down a peg or two. Disdainful mentors, enemies with a superiority complex, arrogant antagonists, and even sometimes heroic types who've gotten too big for their britches. In the ad world, sometimes there's a need for a character that tries to condescendingly make the case for an inferior product, but is shown the error of their ways.
Snotty, completely convinced of their own awesomeness, and enveloped by a feeling of absolute certitude. The condescending voice is all about displaying an air of untouchable superiority.