yeah i agree with ingram87, its for redundancy and faster name resolution
More that one DNS is mainly for redundancy, and *it is* a best practice.
Every network should have more than one DNS server in different geographical locations.
If anything happens at the location of DNS 1 and it goes down, you have DNS 2 to serve your client requests.
You only have one DNS server and the link to it goes down, you’re pretty well hosed. A second DNS server is always a great idea..
DR DR DR DR DR is the main reason.
From the answers already given, you can see it is a good idea to have a second DNS available.
How you do it will depend on your company size. If you are a very small company and are using a peer-to-peer network, use your router as your primary DNS, and your ISP for the secondary.
If you have a real server, not a machine called the server because all your files are stored there on a P2P network, use the server as the primary and your router as a secondary..
Larger still and you may have two or more servers. Use one server for your primary and another for your secondary.You probably will also want to replicate your AD structure to another server as well.
If you are even larger and have another site or several sites, you probably would not need to be asking this question <g>, but this is where you would want to geographically separate your DNS servers. The local server would provide the primary DNS for that site, and the secondary would be at another of your sites.