In GP’s shoes, why bother to learn Mongo then? What’s the benefit? Postgres has a good-enough (if not better) document DB inside it, but it also has a billion

In GP’s shoes, why bother to learn Mongo then? What’s the benefit? Postgres has a good-enough (if not better) document DB inside it, but it also has a billion other features and widespread adoption. Mongo needs a huge benefit somewhere to justify using it.