For reasons of accuracy, efficiency and security many computer network administrators chose to use an external NTP server to synchronize time clocks across their network. It is possible to get a time source from the Internet but these tend to be somewhat inaccurate and certainly a possible security threat. Therefore an external NTP server often simply just makes more sense.
 
Choosing what kind of external time reference you wish to synchronize your NTP server with may influence what kind you eventually buy. There are two options; a specialist radio transmission from a national physics lab or a GPS signal.
 
These are both very accurate time sources, they run less than a few nano seconds off ( a nano second is one billionth of a second) so will be able to provide any computer network with a very accurate timing source. Both do have advantages and disadvantages though.
 
Radio transmissions are based on some of the best atomic clocks in the world, but they are not available in all countries and obstacles as simple as a large hill, as well as unpredictable weather patterns, can block accurate and continuous reception.
 
GPS signals are available anywhere in the world and may be a nano second or so more accurate than radio transmissions. The only problem is that a receiver must have a continuous clear view of the sky to receive an accurate time signal. This can be a real challenge for networks that are located in urban office blocks.
 
There are external NTP servers available that can be programmed to receive both a GPS and a radio signal so that if the reception of one fails the backup source is immediately switched on, avoiding a loss of precious seconds of synchronization.