If you provide a service, you are selling something. If a company has a Web site, they can promote sales through it, and be one step ahead of the companies it competes with (even if they do have a site, make sure you make the company's site better), making it more "stable" in the user's eyes, and thus promoting the service's selling.
A website is also a very easy way to browse information about the services your company offers - most people (especially me) don't particularly like to ring people up etc just to ask about products because it can make you feel stupid, and dependant on other people. Browsing a website you also dont get put on hold as much, and you have control over what you look at when. You can do other things at the same time, and anyone in the world can access it.
An often-overlooked use of websites is a reference for existing clients - information about the business, and perhaps content related to the service the company provides. For example, I have an accountant as a client for whom I have created a site with many articles on tax advice as a client-only resource.
"If youíre not using valid HTML, then you havenít created a Web page. You may have created something else, but it isnít a Web page." - Joe Clark
Websites are very much like how Telephones were for business a long time ago.
Not everyone had them, not eveyone sold things over the phone and there were others that used it and grew their business.
Does every business need to sell and have cart with a SSL. Worry about devlivery of that product or service? NO information is a transaction.
The other examples and reasons above me are all examples of transactions.
If you were to look at my site... Between adwords and a single page that gave me an 8000% return for the month of June. It asks for no CC info only that they call me or fill out the form online and I call them.