Higher quality applications. Fewer privacy violations. Lower chance of identity theft. Better overall security. Longer life cycles for new features. Even longer cycles for critical/security updates. Reduced effort to maintain and keep secure.
On top of that, but not because of the walled garden aspect: Better parental controls. Consistent user experience (although for this one it helps to have the walled garden). Better device interoperability across the ecosystem.
All of that is still irrelevant. You can not have a walled garden and the exact same requirements for the Appstore.
Android is a walled garden too, up until you flip a switch and allow non-appstore apps, which most people do not do. If there were any benefits to the walled garden, they would be present on Android as well.
On top of that, but not because of the walled garden aspect: Better parental controls. Consistent user experience (although for this one it helps to have the walled garden). Better device interoperability across the ecosystem.