
By deploying an enterprise application in a "Software-as-a-Service" format (also called SAS or SaaS), you can insure all members of your company use the most up-to-date version of your application, and you reduce the amount of deployments necessary for future upgrades.
In many ways, this is fundamental to the whole "Web 2.0" movement, because network capacity for the average user, even many mobile phones now, has increased enough that the computer network can be used as an operating system. Because of that increased network capacity, a revolution in application design has started that wiil be as monumental as the first home PC's were in the 1980's.
The pendulum is swinging the other way this time, however, as webservers adopt a role much more like the 1960's era mainframe computers. The increased capacity of the "thin" client today, as well as the high network bandwidth available around the world today, means that many applications will be created that were never before possible.