This sensor is useful for detecting the presence of humans or animals. It is commonly used inside of things like motion-sensitive lights. This is useful when the person you are detecting might not be standing in a specific location. Sunlight will interfere with this sensor, so if you are using it outdoors, it will be unreliable during the day. An example project could be: a light-up sculpture in a forest which only illuminates when there is a person nearby.
See the attached sketch.