They are systems designed to allow machine-to-machine (in our case Mac or PC to web server) interaction over the Internet.
Some of the means of conducting operations (particularly commerce and reservation systems) on the Internet are distinctly flakey. If you regularly book flights or accommodation on the net, you will know what we mean. What would be better, for frequent things at least, would be a method by which we could set up everything on our home computer and then just pass data backwards and forwards to the web service to achieve whatever it was we wanted to do.
If you want third parties, such as other websites, to be able to hook into your systems (with the appropriate security measures in place) then this may be for you. It would particularly apply to theatre box offices and other reservation systems.
At a simpler level, there may be people out there who want to re-broadcast your data to a specific audience. It could be good for business.