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 web address (also known as "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 will be willing to do business with a company that does not have its own domain name.
If you get a domain name that describes your company's business or name, people can remember the name easily and can return to your site without having to consult their documents. In fact, if you get a good name that describes your product or service, you might even get people who were trying their luck by typing "www.yourproductname.com" in their browser.
If you want good sponsors (advertisers) for your website, a domain name is usually helpful. It tends to give your website an aura of respectibility.