Basics first: If I'm trying to access www.superuser.com my Computer first tries to resolve the FQDN to an IP address. If that address isn't in its host file or cache it asks the configured DNS-Server for the IP. In most cases that would be the preconfigured DNS supplied by your ISP.
The ISP then looks up its own cache and let's just say that domain does not exist in the cache either. Now the ISP DNS-Server searches on one of the 13 global Root-DNS-Servers for the DNS-Server for all .com-Domains. The Root-Server replies with an IP and the ISP starts asking this server and so on.. but why is the first part needed?
The Root-DNS-Server only contains IP addresses to all the TLDs. But those aren't that much. There might be 300 to 400 TLDs out there. Why can't the ISP DNS-Server handle those addresses by himself.
So if the ISP DNS syncs its IP addresses to all TLDs every hour or so with the Root-Server there would be a huge performance improvement for each request because the ISP has to send one request less.
Does that even make sense or am I just wrong? Is there a reason for that?