A personal domain name is an inexpensive, internationally universal identifier which gives you more control over your space than other IDs (e.g. email address or phone number.)
On the wiki: Personal Domains.
In order to be able to sign in using your domain name, connect it to your existing identities.
You probably already have many disconnected profiles on the web. Linking between them and your domain name with the rel=me microformat ensures that it’s easy to see that you on Google/Twitter/Github/Flickr/Facebook/email are all the same person as your domain name.
On the wiki: How to set up Web Sign In.
Want to be able to use rel-me data in your code? Check out the open source implementations.