Uchiha Yi himself did not need the Giant Tenseigan to undergo atavism and increase his strength.
But the people around him did!
Since the Ōtsutsuki Clan could use the Giant Tenseigan to undergo atavism, what about the Hyuga Clan? Uchiha Yi had little affection for the Hyuga Clan, but Hyuga Rinka was different. Although neither side had explicitly said so, both the Hyuga Clan and Uchiha Yi himself already considered Hyuga Rinka his woman.
Uchiha Yi felt he needed to help Hyuga Rinka become stronger. She was not like the future Hinata Hyuga. Hinata probably had even greater talent than Hyuga Rinka, but she lacked ambition. Hyuga Rinka was different. If she continued developing as she was now, with Uchiha Yi's help, she might well reach the level of a Kage.
Now, with the Giant Tenseigan's help, Hyuga Rinka's bloodline could undergo a certain degree of atavism as well. Uchiha Yi was confident that he could use the Giant Tenseigan to strengthen her bloodline to the level of these so-called Ōtsutsuki, perhaps even the best among them.
After all, operating the Giant Tenseigan consumed an enormous amount of chakra. Not every member of the Ōtsutsuki Clan on the moon could possibly qualify to be purified by it.
And it was not just Hyuga Rinka. Perhaps Uchiha Mikoto and Kushina Uzumaki would also have a chance to be purified by the Giant Tenseigan in the future.
Once Uchiha Yi noticed that the Giant Tenseigan possessed this ability, he had already made his decision.
I fused with Ōtsutsuki Haguki's physique, didn't I? Doesn't that mean this Giant Tenseigan has been my property since ancient times?
"Why is this happening?" The Ōtsutsuki elder leading the group, a man named Ōtsutsuki Tarō, had originally intended to control the Giant Tenseigan and attack Uchiha Yi.