cPanel has a DNS-ONLY version of the cpanel software. This version allows you to establish a trust relationship between the DNS-ONLY version of cpanel and your other cPanel servers. Once you do this you point one of your nameservers at the DNS-ONLY Version and if your primary nameserver is ever down the DNS-ONLY version will answer querys.

If your nameservers are down then any domain using your nameserver will not be able to be resolved. The end result is end users and servers not being able to find where the domain lives. If you have our sMx service then this will be a problem because if your cPanel server goes down and you have no other nameservers then others machines will not be able to find the sMx Server. Having a redundant nameserver can also save you if for some reason bind fails on the primary server.

This service is run from servers in our facility. By not having this service located in the same facility that your currently hosting out of it gives you better redundancy in case that facility is ever having problems.