Service bureaus provide high-end printing services. Back in the days when desktop publishing was the latest thing and before Kinko's shops dotted the landscape a service bureau was the only place to go if you wanted to output your documents at a high resolution or in better color than the crappy inkjet printers of the time could provide.
Today you'll almost certainly find yourself at a service bureau if you need color negatives, high resolution transparancies or other pre-press output. Service bureaus also usually offer desktop publishing themselves. This isn't cheap but is useful for small to medium sized businesses that don't need a lot of work done.