The Treants are trees with humanoid appearence. The moment their flesh and blood formed, they would turn into cultivators and cultivate just like any other race. There is no difference between them and the other cultivators since their realms and levels were the same as well. The only thing that is different is the final outcome. The Treants did not vie for the Heaven's Will and can't become an Immortal Emperor. Instead, after reaching the apex, the Treant can become a Treefather.
The Treant Race is different from the Demon Race.
The ancestral form is a method of obtaining atavism for Treants. Many within this race would cultivate a human form and become powerful existences. However, a humanoid Treant is not as powerful as an ancestral form Treant, because this atavistic form allowed them to control the strongest origin power.
Despite the great benefits, there is a fatal weakness to the Ancestral Form. There is no way to pass down their bloodline if they were to keep this form, regardless of how powerful and heaven-defying they might be. Only those with a flesh and blood body could reproduce and spread their bloodline to future descendants. Because of this, the majority of the Treants would cultivate into a human form. Very few were willing to maintain the ancestral form. After all, everyone wanted to pass down their legacy.