There isn't really any - it's just a smaller change than banning unsigned apps completely. Their strategy for forcing people to use the app store is to very gradually make it more and more inconvenient not to. This is one of those gradual changes.
Imagine the next ones:
1. You have to enable an option in settings to allow running self-signed apps.
2. You have to disable SIP to allow running self-signed apps.
3. Apps can only be self-signed via Apple's website.
Imagine the next ones:
1. You have to enable an option in settings to allow running self-signed apps.
2. You have to disable SIP to allow running self-signed apps.
3. Apps can only be self-signed via Apple's website.
etc.