This is the first question to ask yourself. If you have found this blog then the chances are you think you may need a website but aren't sure. I am a big believer in the internet and how it can bring you business and increase brand awareness. But what if you don't need new business or want to improve brand awareness?
Imagine you have enough work on to fill your time and bring you the profits you want. Your business comes from word-of-mouth. You work from home, but always visit your customers. Now in this case: no, you probably don't need a website.
What if your target audience doesn't have access to a computer? Or won't use the internet? Again, you have no use for a website.
Outside of these exceptions though, I can't imagine what business wouldn't need a website. (Please add comments below if you can think of anything else!)
At the very minimum I would suggest making your contact details available online. This is almost considered common courtesy for both potential & existing customers, and while there are many directories that will list your details for you, remember you are more often than not surrounded by your competitors or companies with a very similar name. So why not make it easy for people to get in touch with you? They won't necessarily remember or comment on how easy it was, but they'll definitely remember if it took some time - or worse, they'll go elsewhere.
I can list plenty of reasons and uses for websites - but above all else, for most businesses, the key point has been made above.