The bike computer itself can measure speed by the build-in GPS after satellite signal is located.
When equipped with a speed sensor, the speed is from speed sensor,which is the same as the speed from GPS.
The only difference is that the speed from GPS may delay, become lower or lose if riding in extremely surroundings like indoors or in the tunnel.
But, the speed sensors won’t be affected by the external environment.
It’s better to equip a speed sensor if riding at the places where GPS signal is not available, like in tunnel, mountainous area or indoors, or or near buildings.