I think Mongo became popular because it's ad tech and those guys knew how to be buzzword compliant. JSON-esque documents are one thing, but Mongo is Javascript to the core. All of a sudden your JS devs don't have to learn SQL they can just shit out some queries in javascript. Of course that came with some pretty severe drawbacks.
We first used Mongo ~11 years ago with Java. For us the benefit was that we could dump unstructured data into it quickly, but still run queries / aggregations on it later.