Having a backup of your DNS provides another route through which the zone files for your sites can be found. Our DNS backup service copies all of the DNS zones from your nameserver to our server, our backup server is also running BIND, which means that our server can be queried for a zone file if your server is unable to respond for any reason. Which in turn means that the web sites whose zone files were on the that server would not also fail.
The way the system works is we connect to your server once to configure it to allow us to download the zone files via a secured connection, we then store the zone files on our DNS backup server, and refresh our copy every hour. Our backup server is also configured to serve the zone files it has stored. You can then set your main DNS server as your Primary Nameserver and set your Secondary Nameserver as an A record of our DNS backup server, for example, ns1.yourdomain.com resolves to your primary nameserver and ns2.yourdomain.com resolves to our DNS backup server.
If you already have your own nameserver or were interested in having your own nameservers for your dedicated server or for your vps, then it would be a good idea to use a dns backup server, this would in effect provide you with a NS2.yournameserver.com. and so provide you with a fail safe if your firt nameserver were to fail.