Suppose I've purchased example.com from my neighbor's startup domain name company, superlowcostdomainnames.com (aka SlowDNS).
At first DNS is a bit of a performance drag, because SlowDNSs servers are all hosted in my neighbor's basement. My site only got ~100 visits per day, so ISPs often didn't have the domain name record cached. The requests would travel all the way to my neighbor's basement.
But now example.com has taken off and is receiving hundreds of thousands of visits every day. I suppose it must be the attractive combination of rounded corners and a beige background.
So now, most DNS requests are fast, since they're cached at the ISP.
But a representative for SkyFlashCDN is trying to sell me their distributed DNS service. Instead of requests traveling to my neighbor's basement, they'll be serviced by "one of 1138 data centers around the world."
My site is popular, and ISPs are basically functioning as a distributed DNS service. Why would I pay for the service when I'm (apparently) getting the equivalent for free? Is there some performance benefit I'm missing?