It's more about providing a basic level of service to the marginalized groups in our society who are unbanked. It also provides a bulwark against extortionary and predatory pricing. Take check cashing as an example. Every time you get paid from your (I'm assuming highly paid) tech job does your bank take 1%? That's the story for people who don't have bank accounts. You and I would never use these services.

Many countries have postal services offering banking to help pay for the post/because it works well with postal branches everywhere. They are separate divisions and the banking division is run like a bank. But I am always skeptical in the US, because government employees and caring, efficient/competent service don't seem to mix all that often.