1. Domain names are your Internet address. Most domains are registered by business as a way of providing information and a means of communication to their customers. Many businesses register their domains in multiple countries in order to protect their brand. Owning your own domain also allows you to have a personalized email address and website if purchased with hosting. Rather then using generic emails like hotmail and gmail you have setup, you can have an individualised email e.g. joe@joesbuilding.com.
2. Domain names are portable. With your own domain name, you are no longer tied to your ISP to access email or host your website. No more changing email addresses when you change ISP's!
3. Domain names improve search engine rankings. Search engines rank websites with their own domain names higher than those using an ISP address.
4. Domain names show professionalism. Why advertise a website address like: http://www.yourisp.com/user123/ when you could have www.yourdomain.com. Your customers will be more confident dealing with your company if you have your own identity on the internet for your website and email.
5. Domain names are memorable. A long website or email address at your ISP can be difficult to remember. A short, relevant domain name is memorable and will increase traffic to your site.