In short, it is the treatment of the Son of Heaven and Earth, the higher the oath, the greater the wishes, the stronger the effect of the aid from Heaven and Earth.
Take Yuzhao for example, who swore an oath to exterminate demons, with the grand vow that she would not become a celestial being until the Demon Catastrophe had ceased; the fate bestowed upon her was indeed terrifying. Before reaching the Tribulation Realm, even before becoming a True Immortal, there would be no bottlenecks hindering her progression.
That is the benefit of the Great Oath of the Alliance Heavenly Tao.
However, with the good comes the bad; the disastrous consequences of such a great oath and great wish are equally apparent because the essence of this vow is to borrow power from heaven, and as much as you borrow, you must repay. If one fails to achieve, the karma would be a hundredfold and certain annihilation would ensue.
