1. It's to provide protection in case of neutral failure. If you have an appliance with the case grounded via neutral and the neutral wiring fails, you now have the case connected to live (through any appliance that is switched on). With separate ground, nothing happens as you are only allowed to connect ground to neutral at a point where failure of the upstream wiring is practically impossible (i.e., wires so thick that they practically won't break).
2. It's to provide a path for ground fault currents that does not pass through the RCD, if you have an RCD. While neutral doesn't go through the circuit breaker, it does go through the RCD, so as to measure the difference in current between live and neutral. If your appliance is grounded vial neutral, any ground fault (connection from live to the case) looks like regular load current to the RCD, if it is grounded via separate ground, the fault current bypasses the RCD and thus causes it to trigger, even for a small current that would not trigger over current protection.