Is the saving of mankind's few, by creating a new, higher reality, the reason you're here on Earth?
One of them, yes.
They left to go back to their other homeworld (Altea).* They had worked diligently to teach the way of higher civilization to groups of humans all over the world, so that the few that would survive would start life anew after the cataclysm, and then they left.
Why didn't elevating civilization work to save them?
They knew it wouldn't save the majority (70%) of mankind, but they tried to use the event to jumpstart man into a higher level of civilization so that next time (our time) they would be beyond the reality of such a calamity.† The Original Ones could see the value of the presence of humanity among the star races. But they did not yet know how powerful proxy magic could be to help ensure the continuation of the race.‡
How did the Original Ones manage to learn about this powerful tool?
The Holy Mother taught them.