For what purposes are triac couplers used?

Triac couplers are intended for heater control circuits for industrial, home electronics, and other purposes, as well as SSR (solid state relays). They are used for performing ON/OFF control on a load connected to an AC power supply.
For details, also refer to Toshiba Application Note "Triac couplers-basic properties and application design", section 11 Design Example for Triac Coupler.
(https://toshiba.semicon-storage.com/ap-en/semiconductor/knowledge/application-note.html)