Whether you have a domain name registered or not doesn't matter. These services provide DNS functionality and features. Most domain name registrars only provide rudimentary DNS service (ie. you input your IP, or you enter the DNS entries for your host) whereas these services allow a wide variety of things, from "generic-name.domain.com" to point at your PC, to a domain name pointed at your pc, to MX setup for DNS to run a mail server, to web hopping (EveryDNS does this, and this would be used to point somewhere like www.geocities.com/blah/blah) etc. etc. Not to mention the ability to automatically update your IP address if your ISP re-allocates IP addresses.
Incidentally, EveryDNS has renamed themselves to EveryBox, expanding (or planning on future expansion) into further services. You can reach them at www.everydns.net still, or
http://www.everbox.com. These guys are definitely worth a look...
/L.A