Marten as Document DB
Marten's original focus was on enabling Postgresql as a document database for .Net developers that would allow developers to work very efficiently compared to the typical RDBMS + ORM approach while delivering solid performance. In Marten's case, persistent .Net documents are just serialized to JSON and persisted to Postgresql using its unique JSONB capabilities. The advantage to developers of this approach is that there is much less configuration work necessary to map their persistent classes to a database structure. Developers are also far more able to evolve their application's model as there is so much less friction in changing the persistence layer with Marten compared to the RDBMS + ORM combination.
Here's an introduction to Marten Db as a document database from .Net Conf 2018: