Zero crossing detection and Light dimmer
Devices such as the MOC3041 will only turn on as the supply goes through zero - this is desirable for controlling loads such as heaters or motors as it can reduce the interference they cause and avoid getting partial cycles. The load will be either fully on or fully off.  Any PWM that is desired can only be performed at a very slow rate - this is acceptable for heaters where the response time may be many minutes.

The load can only be controlled for complete cycles (or half-cycles) of the AC supply - this is not what is required for controlling the intensity of a lamp by using phase control where a fraction of a cycle is desired.  The lamp would flicker unacceptably