There are a number of good reasons for having a domain name:
- If you ever change your web host, your domain name goes with you. Your regular visitors or customers who knew your site name as www.yoursite.com (for example) would not have to be informed about a change of URL. They would simply type your domain name and they'd be brought to your new site.
- If you are a business, a domain name gives you credibility. Few people would be willing to do business with a company with a dubious URL like http://www.geocities.com/whatever/12345.