The gyroscope sensor in a smartphone is used to detect the phone's orientation. It measures the phone's angular velocity and can automatically adjust the screen's orientation to match the phone's physical orientation. This allows the phone to be used in any orientation, whether portrait or landscape. Some apps also use the gyroscope sensor to track the phone's movements, such as in augmented reality games. In general, the gyroscope sensor allows the phone to be used in various ways and provides great flexibility for app developers.