A sort of celestial NOAH, he repopulates the world with humans after major disasters.
Apparently he once looked after VISHNU when he was a tiny fish, and nurtured him to full fishliness. Afterwards, VISHNU repaid him by warning him of the mighty flood and how he could be the only being to survive it.
Apparently there are supposed to be fourteen wash-it-all-away floods before the world ends. Each time it happens, MANU will be there to man the boat.
So far we are about half way; the floods can be millions of years apart, so there is no need for immediate panic.