It's fortunate when working on the Web that decisions are rarely final. You might think your site is going to have a limited audience, certainly not worth the expense of registering a domain name. Surprise! You suddenly find yourself one of the Internet's hot spots. You start thinking about the advertising revenue possibilities. And if you're going to get paid by the eyeball, you want to ensure that you have lots of eyeballs. Suddenly, you find yourself in need of an easily memorized domain name, after all.
Or say you were only planning a simple site with a few pages. A couple months down the road, you decide to add a message board to your site. The problem is that your ISP does not allow CGI or PERL. Guess what, it's time to move to a new Web host. And while you're at it, you decide you might as well register your own domain name to ensure you take your regular visitors with you the next time you switch providers.