I believe one of the main security benefits of these requirements are ensuring a binary hasn't been altered maliciously, or otherwise I guess. Not preventing outright malicious applications.
Also so the os doesn't have to repeatedly rescan apps, etc.
sha256sum generates a sum, you still need to store that sum somewhere that isn't controlled by the malware creator or they can just change the sum too.
All major Linux distros for example still have no viable way of creating signed programs or anything like Gatekeeper.
Malware authors frequently sign their applications with valid Developer ID certificates. This change is not something that would keep them out and I strongly suspect that was not the goal when it was made.
The key here is that Apple can also remove and retroactively revoke signatures on signed applications, which is why you rarely see Mac malware stick around after it makes the headlines these days.
When a piece of (signed) malware becomes even moderately sucesful, Apple shuts down all its installation vectors by banning every dev account that's ever signed it. If nothing else this makes it harder to develop malware that spreads rapidly through the App store, forcing bad actors to invest a lot more work into finding 0-days.
If you have a vulnerable app, it might modify another already approved app that is already installed, thus omitting the scary message about untrusted code before being run.
