Once you get your web site up and visitors start coming, you'll wonder what you ever did without it. Depending on how much maintenance you do on your site in-house, a web site probably costs considerably less per year than a postal meter. As far as the customer knows, you could be out of business.
Web sites can go down for any number of reasons, and sometimes for good ones.
Web servers are just big computers. They need occasional maintenance any, like any other machine, they can break down. Usually the maintenance is scheduled in the wee hours of the night and takes just a few moments. Occasionally something unexpected comes up during the day, but can still be handled quickly. It may be that one site on the server experiences an unusually high volume of traffic, or a virus strikes, or a site owner uploads a script with bugs that temporarily overloads the system.