Why is zero crossing detector is used in some AC control circuits?
Zero cross switching can be used so that the switch action occurs when there is no voltage across the load and thus no current through the load. This prevents fast rise time current flow as what would occur if the switch closed or opened when the voltage was high. Eliminating this can:


Lower stress in semiconductor components that are used for the switches.
Reduces stress in capacitors that may be in the circuit so they are not asked to charge and/or discharge at a very high speed.
Reduce EMI (electromagnetic interference). EMI can occur when voltage sources are switched to and from their loads and resultant current transients can cause large amounts noise of noise be induced into nearby circuits. Zero switching eliminates this.