Why are optoisolators often used in conjunction with solid-state circuits that control 120 VAC circuits?

Optoisolators (also known as optocouplers) are used to transfer electrical signals between two isolated circuits by using light. In the context of controlling 120 VAC mains power with low-voltage solid-state circuitry (like microcontrollers), they provide critical electrical isolation. This isolation ensures that a catastrophic failure, short, or voltage spike on the high-voltage 120 VAC side cannot flow back into and destroy the sensitive, low-voltage control circuit. It essentially acts as a protective barrier, coupling the signal optically rather than electrically. The distractors are incorrect because optoisolators do not provide a low-impedance link or impedance matching; in fact, their primary purpose is to provide near-infinite impedance (galvanic isolation) between the input and output.