The reason Qin Jun waged war against other kingdoms was to accomplish the main quest of Slay Hundred Emperors. Upon completing the task, his level would rise by four, and he would receive one opportunity for a God and Demon summon, one random draw chance, one skill inheritance chance, and one chance to summon a God and Demon legion.
The rewards were indeed immensely generous.
Of course, slaying a hundred emperors was an overwhelmingly huge undertaking.
It was uncertain if the Righteous Dao even had a hundred kingdoms...
"War begets war. Do you still not understand the situation Great Qin is in now? We must grow stronger, and quickly!" Qin Jun declared somberly. When could true strength be amassed if not by nurturing one's energy and biding one's time?
War signifies resources, and resources drive development.