This is a great question, and before you get started, it's a good idea to be able to articulate your answer as briefly as possible. For example, you might say "I need a web site to showcase my artwork, so I can sell prints online." You may say " I need a website so that people will understand how my service contracts work" , or you may say " I need a website to give accurate directions to my business since I am located out of the way".
You get the idea. The important thing is not to think you need a website just because everyone else has one. Make sure it serves a purpose, and you can articulate that purpose, so you dont waste time and money putting it together!
Remember things like interactive maps, galleries of photos and images, downloadable forms and brochures, and of course shopping carts will all do your work for you while you are out drumming up more business.