Answer 1

XML is a tree by design. So if you have a very hierarchical structure xml can be useful for clarity. I guess, that is why the big GUI frameworks use it to describe windows.

Answer 2

First thing that comes to my mind:
With xml you can validate your document against a xsd. So that may be the better choice if you want to exchange data with a second party.

Answer 3

Advantages:

XML, older and more established. slightly more human-readable.

JSON, more concise, faster.

Circumstances:

dictated by whatever frameworks and APIs demand.

Answer 4

Sharing data across different platforms when json is not an option.