Most administrators keep all their DNS servers on their own networks, under their own control.
Some companies make money providing third-party DNS service.
These companies claim that third-party DNS service provides huge benefits. The purpose of this web page is to analyze the actual costs and benefits of third-party DNS service. The bottom line is that, for the vast majority of sites, third-party DNS service has serious costs and negligible benefits. The service companies' claims are wildly exaggerated, and should never be used as a substitute for common sense. They need to locate a suitable third party, to arrange zone transfers, and to track changes in the name or IP address of the third-party server.