In audio systems it's sometimes desirable to have one or more channels out of phase with the other channels. In practical terms it means a delay between a sound being played on the front speakers of a Surround Sound system and the back.
The big advantage to having the Surround channels out of phase with the front channels is that they don't interfere with one another. Since surround speakers may be pointed nearly directly at front speakers they'll often have a tendency to interfere with one another if fired in phase. By having the signals slightly out of phase (generally 90 degrees) the risk of interference is alleviated.
