In this particular example you need hierarchy for on at least two occasions.

1. The xml denotes that the whole thing should be wrapped in a div, and shouldn't just be considered a paragraph. With only lists you can't denote what kind of level you're at, unless you force each level to always carry the same meaning (which makes the format weaker than XML).

2. Setting "bold" to true isn't the same thing as emphasizing the text. Typically 'em' is italic, but more importantly a nested 'em' should no longer be italic. To achieve this with JSON you're forced to combine formatting and structure in a way that CSS was designed to prevent.