|YOU'VE FORGOTTEN YOUR PURPOSE!
The Past is the timeline, Where Jack and his father reside and living peacefully until Aku wreak havoc. It is also Jack's main objective needed to go back after Aku had trapped him at the alternative future after his future self had conquering the whole planet.
At the time when Jack was in the past before clashing with Aku, earth had a wonderful physical structure, later being damaged by Aku and his minions. Jack intend to find a way to get back to his home.
When Jack returned to the past and destroyed Aku for good, the world was saved from his tyranny. This extended to the future as well since there were no Aku to turn it bleak.