Probably because it allows them to extert control over their hardware even when users hold it in their hands. Given System Integrity Protection this allows them

Probably because it allows them to extert control over their hardware even when users hold it in their hands. Given System Integrity Protection this allows them to enforce rules on the client and trust them on the server. For example shipping apps from the app store that can't be pirated and trusting that iMessage clients are actual hardware owners with unmodified clients. It is basically a DRM system.