This system has its Z axis parallel to the Earth's rotation axis (positive to the North) and its X axis towards the intersection of the Equator and the Greenwich Meridian. Thus it is convenient for specifying the location of ground stations and ground-based experiments as these are fixed quantities in the GEO system.
When GEO coordinates are expressed in spherical form, the latitude component is identical what is termed geocentric latitude by astronomers and geographers. However, note that this is different to the system of geodetic latitude used in normal map-making. The geodetic latitude at any location is the angle between the equatorial plane and the local normal to the Earth's surface. In general that normal is NOT parallel to a radius vector because the shape of the Earth is an oblate spheroid and not a sphere.