1) The internal or wire representation of data from document DBs isn't necessarily JSON, though it's normally converted to such.

2) "Document" has undergone a bit of semantic drift thanks to HTML and XML. In an informational context it means "structured, hierarchical unit of data containing mostly text". The data from forms, invoices, and the like needs to be collected and stored, even if it isn't properly normalized and relationalized (or is en route to being such) so a "document database" is thought to be suited to this task

I dunno, whatever, I'm in the "just fucking use postgres until you can justify why you shouldn't" camp.