Optocouplers are used mostly in power electronics circuits to isolate the fatal mains utility voltage to that of the low dc voltage that the control circuit works. Therefore some call it optoisolators. They find use in control circuits feeding Thyristors,MOSFETs,IGBTs. Submitted below an audio visual for better understanding.
Controlling AC power by delayed firing angle control is a very old and traditional method. Simple example is our electronic fan regulator. But this has one major drawback that it generates a lot of harmonics (62%THD)as compared to a chopper(2%THD). Presented below a chopper that is much more efficient. . Please watch the video