This is a very good question, and one which has a very simple answer!
In brief, a website is a marketing tool, which will advertise your business 24 hours a day, 7 days a week, 365 days of the year. Even after your business or organization closes, people all over the world can access your site and see what you have to offer.
A website will also help you acquire new and potential customers and clients locally and globally. I have prepared a document which you may find helpful in answering this question, which also acts as a start up and resource tool.
If you would like to see a copy, please get in touch, so it can be emailed to you.