Among other things, determinism, since timing can be guaranteed (within a margin). RTOS will run with consistent timing, which is guaranteed and facilitated by control over tasks priority and checks whether timings are met. You could probably do that without a hard RTOS, but without any sort of formal guarantee. So, it might work all fine and dandy, until it doesn't. Doesn't should not exist in a hard RTOS by definition and proofing of it.