The Majin Buu Saga, Dragon Ball Z's final chapter, saw Goku and the Z Fighters battle the titular monster, who developed into several forms before eventually being split into two halves, one good and one evil. Kid Buu, the evil half, was eventually defeated by Goku's Spirit Bomb and reincarnated into Uub, making him a loyal friend of the Z Fighters.
Good Buu is still the most powerful being on Earth apart from Goku and Vegeta (and Arale) and remains the most powerful being on Earth in terms of power. Yet for all this power, he remains remarkably underused throughout Dragon Ball Super and GT. Buu is still far behind other less powerful fighters like Gohan, Piccolo, and Krillin in terms of his time in the spotlight.
Buu is Too Broken
Buu is a legendary Dragon Ball character who was once considered one of Frieza's worst enemies in Universe 7, owing to his virtual immortality. Piccolo and Frieza also use this ability, although slower, to absorb damage.
Buu is even more difficult to defeat than Goku or Vegeta, and it's still difficult to imagine him losing a single arc to Super's strongest villains. After this, Buu sat out several seasons before being invited to join the Z Fighters in the tournament of power. He even exhibited a new, slimmed-down physique for the occasion, which he used to spar with Goku.
After all the planning, Buu fell asleep and was replaced by Frieza for the tournament. It's as much a gut-punch to fans as it was to the Z Fighters. However, if we ask why did Super's writers chose to have Buu absent from the Tournament of Power, the only answer is that he felt his presence would rob Team 7 of its underdog status and reduce the overall stakes.
Even in retrospect, it's hard to imagine all but Universe 11's Top and Jiren defeating Buu, and even then likely by ringing out rather than damage. The truth is that Buu's regeneration makes his battles inherently one-sided against all but the strongest warriors in the cosmos, and the writers usually want to save those conflicts for the Saiyan duo.
Buu's Potential Redemption and GT
The Dragon Ball Super anime ended after the Tournament of Power, but the manga is still going on, and Buu got a significant lore drop during the Z Fighter's confrontation with the planet-consuming Moro. While it was already known that Kid Buu became a result of the latter's consciousness, it was revealed in this arc that the latter's consciousness remains inside Good Buu, and is capable of taking over in various situations.
Buu is a far more complex and interesting character than he otherwise would have been, and is open to more powerful cosmic threats. Of course, Goku almost always has to be the ultimate victor, but it's worthwhile to see Buu finally utilized for more than a few brief fights or comic relief in recent Dragon Ball Super: Broly chapters, and it's unknown if he'll feature in Dragon Ball Super: Super Hero.
Buu eventually joins Uub and becomes Majuub, which in reality seems to be Uub with a power boost, and his genesis seems to be a de facto death for Buu. Despite his insanely broken abilities, Buu deserves to stay around and be expanded on.