[william-shulman]

I agree with the other reasons posted. One more reason is one of expressiveness. When you have a "flat" simple form, it is easy to serialize as name/value pairs and send to the server in that fashion. However, when a form is more complex, with nested structure or with variable-length lists of data, it can be very difficult to come up with a serialization scheme that fits the name/value pair model. Often you end up creating your own mini language. If, however, you use a JSON object or XML node as your payload, you are free to transmit data of arbitrary complexity rather easily. So in cases where the data you want to POST/PUT is not flat, you will have much more expressive power encoding your data in JSON or XML and posting it in the request body.