Let's say you need to redirect a website within your domain to an external host. Instead of creating a webserver and redirecting the traffic. In some case such as being on the same domain and wanting to redirect to an external name yet it includes your domain.
example: your domain is example.com. You want www.example.com yet it is hosted by somewhereelse.com. somewhereelse.com has a c-name of www.example.com
Solution is to add a delegation under your forward lookup zone. Add two new name server: resolver1.opendns.com and resolver2.opendns.com