Usually the reason for including the atomic clock is reliability: the atomic clock can keep good enough time for a short period if the GPS signal is lost. Their

Usually the reason for including the atomic clock is reliability: the atomic clock can keep good enough time for a short period if the GPS signal is lost. Their time cards still primarily run off of GPS (in fact there are 3 nested clocks in the standard design of these systems: an oven-controlled crystal oscillator which is already quite stable by itself, but more importantly gives a very clean sine wave output, then an atomic clock (usually rubidium) which is more stable but higher noise which is used to tune the crystal oscillator, then the GPS signal which is even more stable with even higher noise used to tune the atomic clock. You can often just skip the atomic clock in the arrangement and you'll get the same performance while you have a GPS signal.)