I'm not sure what you mean. Do you know what a zone transfer is? If you wanted to get a list of the domains and records published in a nameserver, you would perform a zone transfer. Because that can amount to quite a bit of information being transferred, if a nameserver allows unrestricted zone transfers, that's a vector for a denial of service attack against that nameserver.
If you're a domain registry, your zone files are huge. Allowing arbitrary zone transfers could potentially put massive sustained strain on their DNS infrastructure. And thus because only a very small number of nameservers really need to be able to perform zone transfers against their nameservers, they're better off locking down the ability.
If you're running your own nameservers, then it's still worth locking down zone transfers for similar reasons. At the very least, it gives you a degree of defence in depth as you're giving attackers less of an opportunity to gather information on the structure of your network. If they could simply do a zone transfer to find out all the names in a given zone, then they don't have to do more costly brute force enumeration to guess at the hosts in the zone.
Take a read of this for why, if you run your own nameservers, you shouldn't allow arbitrary zone transfers: http://www.iodigitalsec.com/dns-zone-transfer-axfr-vulnerabi...