While the kernel has a large amount of userland-reachable functionality, much of this attack surface is not accessible due to sandboxing in iOS. By default, an app is only able to access about 10 drivers’ userclients, which is a relatively small amount of code. Therefore, first escaping the app sandbox can be highly beneficial in order to attack the kernel.

In contrast to the kernel, many daemons running in userland are accessible via the default app sandbox. One such example is a daemon called MIDIServer (com.apple.midiserver). This daemon allows apps and other services to interface with MIDI hardware which may be connected to the device.

source: R