LDRs (photocells) are used for switching. They have a negative coefficient wrt to incident light. The LDRs used in amps are co-packaged with an LED. When the LED is on the LDR will pass signal and vice-versa.
LDRs are popular because they are fairly silent when switching, last long and can withstand high voltages. A possible downside of an LDR is that the resistance is also dependent upon applied voltage so they will add some distortion. Whether or not that distortion is undesirable is open to debate.