Note that currently, the Solid spec mandates both RDF/Turtle and RDF/JSON (JSON-LD). That said, it's perfectly possible for an app to read/write any data format

Note that currently, the Solid spec mandates both RDF/Turtle and RDF/JSON (JSON-LD). That said, it's perfectly possible for an app to read/write any data format they choose, but RDF deals with some challenges regarding data discovery and interoperability, and allows for more granular updates with smaller payloads. (I also work at Inrupt, though views are my own.)