
https://www.npr.org/2017/01/10/509126878/what-is-driving-the...
For a poor person, a check-cashing place is generally the cheapest and most straightforward way to cash checks (and receive other basic financial services) with clear, reasonable pricing. If you're poor, acquiring the same services from a bank is not only vastly more expensive, it's also much less transparent in terms of how and when fees will be assessed.
Often this confusion stems from people who aren't that familiar thinking they're the same as payday loan places, which are just legal loan sharks and designed to trap people in an endless cycle of debt.
None of this is anything against postal banking, which I'm hugely supportive of.
For most poor people, checking cashing businesses are both cheaper and more transparent than any other way of getting their checks cashed. This has been studied - they would pay more (and in much less transparent fashion via a series of fees) if they went to actual banks.
I recommend reading The Unbanking of America by Lisa Servon to get a real understanding of the industry.