It's short and memorable
You can have a website without having your own Internet domain, but your web address will be something like:
(Try saying that fast three times.)
But, with your own domain, your address will be something as simple as:
It's your "identity" on the web
Beside being easy to remember, having your own domain is important, because your website address is your identity on the Web. It tells people who you are and, often, what you do.
Some familiar (and effective) website addresses are cnn.com, microsoft.com and webworkers.com.
It's permanent and consistant
This allows you to keep the same address and print it on your business cards and literature, without worrying about having to reprint, if you later change your Internet service provider or website host.
It says you're serious about online business
Your own domain and domain e-mail address tells your present and potential customers that you are looking to the future and that you're serious about doing business online.