22kHz maybe when you are a baby, but the range of an average adult is FAR lower than that. There's two reasons you might want to have a tweeter that is capable of extension far greater than you can actually hear. A conventional tweeter should act as a perfect piston. You want a tweeter that is comfortable at 20kHz, not nearing its breakup point. If a tweeter is in its nominal range at 20kHz, it will probably be able to respond at far higher frequencies.
Second, supposedly UHF capable ribbons or super-tweeters have the ability to "energize" the room in a way that is more felt than heard, like the effect of sub-audible bass, except at the opposite end of the spectrum.