Imposing more structure on the data types/schemas supports computation, whereas imposing less structure is generally better for presentation purposes.
In general, it is possible to convert structured data to unstructured, and vice versa, but there are many, and often sub-optimal, transrepresentation strategies.
Additionally, there are multiple different ways to structure data (e.g. frame/schema based, graph based), and multiple ways to capture unstructured data (e.g. 'notes' in different natural languages, charts/graph(ic)s/diagrams)