Pierre regulates the lights, according to motion detection
These sensors detect human motion but they are not suitable for detecting stationary humans. The best usage is in halls and places where people don't spend too much time in one place. Mostly it's used for lighting automatization.
When you walk by motion sensors in the hallway, lights go on, and as soon as you pass them by, they turn off. This way Pierre saves energy because lights work only in case of motion detection.