My specific use case was building a form where each change to an input would fetch a new copy of the form from the server and morph it in place.

It means the server-side code can be really simple. You can make parts of the form depend on the values of other parts. For example you can show/hide a section based on a checkbox or fill a select with options based on a previous selection.

Because it was a form, it was really important to maintain object identity and state perfectly so the user would not be interrupted.