There is no precise answer to this question. There can be a hundred reasons to have a Website, and a few important once might be the following.
First a Website can reach millions of people that might not otherwise be available. Second, customers can view products, services, or whatever you want to share from the comfort of their own home. No matter what type of business you are in, there are ways to grow and make money on the web for you.
Believe it or not, a Website is the least expensive way of getting your message out. When you consider that 72% of the World public has Internet access, it is always accessible and can carry a message in just about any fashion you can imagine, the Internet is the least expensive way to market your company.