The Garmin Speed/Cadence Sensor (GSC10) and Bike Speed Sensor (BSS) override the speed and distance data from the GPS satellites.
The GSC10 uses a magnet on the wheel and the bike profile's wheel size to calculate distance and speed with improved accuracy, while the BSS uses its magnetometer and orientation to accomplish the same thing.
These sensors are useful when traveling through tunnels, areas with heavy tree cover, and on rides with substantial changes in elevation where GPS is limited. When the device is recording distance with GPS, it is recording the distance from one point to the next, and may not be factoring in the elevation ascent or descent. The sensors give a more accurate reading by recording distance based on wheel rotation.
If GPS is preferred for speed and distance, the magnet on the wheel (for GSC10), or the entire BSS will need to be removed.