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The ability to retain a JSON document opens many possibilities for downstream processing and eventing. As a document posted to the API represents a "one logical event" in the operations of an school district (e.g., "student X was marked absent on day Y"), the pre-packaging of that data open opens up the possibility for other data consumers to consume it as a documents (e.g., the document could be posted to a log of attendance events to which other systems subscribe).
The relational format of the ODS data storage delivers other benefits, such as the ability to perform complex validations based on SQL, so it is is not a case of one storage format is better than the other, but that there are use case benefits to each. Indeed, there are also certainly ways where both technologies could be mixed.