1. Your customers expect to be able to find a solid representation of your business on the Web. 
2. You will gain credibility. A professional, well-designed site makes your business look successful, and builds trust in your company and its products/services. New customers often judge your business by its outside (online) appearance. 
3. Web technology is ever-changing. Web users get frustrated by antiquated Web pages--unnecessary animations, multiple frames, static content, and the lack of new features such as blogs, feedback forms, and online stores are just a few things that may irk your site's visitors. 
4. Search engines may be skipping over your old site because it is not optimized to be included in common searches. 
5. Old, ugly site with old content = no new business coming from Internet users.