The Main Plane was where Zhou Tian's foundation lay. Although Zhou Tian was no longer following his old path at that moment, the strength of the Human Emperor was something he dared not lose. Without the Human Emperor's strength, Zhou Tian knew what his end would be. Thus, he was very concerned about the power of the Human Emperor, and under those circumstances, he needed to protect the power he held in his hands.
Clearly, if Zhou Tian did not want to lose the position of Human Emperor, he had to participate in the imminent war on the Main Plane. Moreover, in the battles to come, Zhou Tian might not aim to gain much, but at the very least, he could not afford to be at a disadvantage against those opponents. Otherwise, even if he participated in the coming war, if he suffered heavy losses, he knew his position as Human Emperor would also be hard to maintain.