Alignment and calibration are not the same job
An alignment sets the wheels. Calibration tells the camera and radar where straight ahead actually is. On many vehicles a rear thrust angle change from an alignment moves the camera's reference, which is why the two jobs are increasingly linked.
When it is required
After windscreen replacement on a camera-equipped vehicle. After suspension or alignment work on many models. After any collision repair affecting the front of the car or the sensors. Sometimes after a battery disconnect, depending on manufacturer.
Why it matters
A camera aimed a fraction of a degree off can put automatic emergency braking's picture of the road in the wrong place. These are safety systems, and a miscalibrated one is worse than an absent one because you trust it.