Here's a big claim: relational databases are the best primary data-store available for 90% of the apps you'll write. They're a great basis for fast, reliable backends that scale to millions of users.

Why?

Relational databases do a lot for you - like enforcing data quality and making lookups efficient. They're incredibly flexible to query, meaning your database designs will support new features without change. And Postgres specifically has advanced features like streaming and publish-subscribe that work fantastically with NodeJS.