To keep ahead of your competitors. By registering your own domain name and using a personalized email address you will be one step ahead of your competitors and will impress your clients and suppliers with your professional presence on the Internet.
A domain name allows your customers to find you at your unique root address on the WWW. Using a domain name will make your web site more accessible to your visitors whilst giving you a more professional profile.
You may wish to register a particular domain name in order to secure it for future use (before someone else does). You should secure a good domain name now even if you are not ready to use it. Nearly 17,000 names are registered every day. Any delay may result in you losing out on your ideal name and having to settle for something less appropriate in the future.