They're useful for doing more on the client than server. If your UI runs only on the client then you just have to write a web service to communicate with. You j

They're useful for doing more on the client than server. If your UI runs only on the client then you just have to write a web service to communicate with. You just take the V out of MVC, or you have views that simply emit JSON/XML/whatever. Most web pages would not benefit from this but many web apps can and do.