These kinds of in-process/in-memory versions of dependencies take the startup time of those test from minutes/seconds to milliseconds, allowing you to run your tests vastly faster. That's a game changer for developer productivity.

What's great is that your code still just depends on "postgres", so you can test against this in-memory version most of the time then occasionally (such as in CI) run that same suite but either a "real" postgres as a way to make SURE you're not missing anything.