Crossover distortion (Wikipedia:) "is a type of distortion which is caused by switching between devices driving a load, most often when the devices (such as a transistor) are matched. It is most commonly seen in complementary, or "push-pull", Class-B amplifier stages, although it is occasionally seen in other types of circuits as well."

The typical harsh and gnarly sound in not appreciated by most musicians. But for some genres like noise and drone it is perfect to put some dirt in the sound without killing the amp.