Heroku already provides a managed database service using Postgres, so you're fine for that.
It's if you want to store any kind of user uploaded or generated binary or large content - images, PDF reports, audio files, videos, JSON documents, code, logs - all common things you may want to manipulate in a web service.
It's if you want to store any kind of user uploaded or generated binary or large content - images, PDF reports, audio files, videos, JSON documents, code, logs - all common things you may want to manipulate in a web service.