This is one part of a range of alternative switches, often used in place of a proximity detector or to ensure safety when an obstacle may cause a location to be unsafe, eg when a door way of a corridor may need to be clear before a door opens.
We use them occasionally on the insides of gates to enable egress, so that on approaching the gate will open, thereby ensuring that a person propelling a wheelchair can maintain progress without having to stop to operate a switch.
When the beam is broken the detector is operating, thereby sending it’s command to the door opener or alarm.