Just as in autumn on a cloudless day the image of the moon can be seen reflected in a pond, so the disciples of the Buddha perceived the manifestation of the omnipresent reflected in the body of the Buddha.
If the Buddha-Nature did not exist there would be no impulse to be liberated from the suffering of samsara and no heartfelt attraction to nirvana. Samsara is full of suffering and is therefore impure. Nirvana is bliss and therefore has merit. Neither would exist without the Buddha-Nature.