Domain names aren't worth much in themselves. For someone who wants to supply services, they can start out as just part of the price of entry to really participate, but they also typically become a sort of brand name. Behind it all, you need a machine that can supply the services that are the life-blood of the Internet.
With my main machine, I can run many services. I run a web server and an email server. I also run my own DNS server so I can tell the world where to go instead of relying on someone else to do it for me.
You sometimes hear these days of "hosting a domain". Really, a domain will often have several hosts in it, some of which will act as servers. When someone speaks of hosting a domain, they generally mean they provide a few of the standard services off of one server, with the DNS system set up to point everything to that server. Under the DNS system, many FQDNs can map to the same ip address. In fact, there's practically no limit to the number of "web addresses" that can map to an ip address. This is how I host so many domains. In fact, I even cheat a little more. Some of the domains I host map to different ip addresses, but those addresses currently all belong to the same machine (using a techinique called aliasing).