Primary DNS and Secondary DNS
The DNS service enables you to type names like en.kioskea.net instead of writing the IP address of your machine whenever you connect to the computer.
To achieve this configuration, special servers are used. Every computer who get connected to the internet is configurated with a list of servers which is automatically filled in by your ISP.
The secondary DNS as a back up for the primary DNS in case it does not work. There is no guarantee that the first DNS work at all times.
The above stated reasons clearly refer to why all domain should imperatively have 2 DNS server with a fixed IP addresse.
For more information about DNS, follow below link:-
http://en.wikipedia.org/wiki/Domain_name_system