Why did Bibidi seal Buu?

Question

Bibidi sealed Buu away after Buu absorbed the Grand Supreme Kai. The Dragonball Wikia page for Majin Buu notes that he becomes easier to control after he absorbs the Grand Supreme Kai:

Of note, absorbing Grand Supreme Kai granted Majin Buu the ability to speak fluently, although he usually refers to himself in the third person. Buu became calmer and easier to control after absorbing the Grand Supreme Kai, and Bibidi then decided to seal him in a Sealed Ball.

Why would Buu need to be sealed up if he had become easier to control?

Answer 1

Bibidi did not seal Buu right away. When Kid Buu absorbed Grand Supreme Kai, Buu's personality and behaviour pattern changes. Along with the ability to speak, he brings out a childish innocence. Bibidi took advantage of this innocence and brought Buu under his control.

From Wikia article on Bibidi:

However, as Buu tends to become disobedient from time to time, Bibidi devises a Sealed Ball that can be used to temporarily trap the monster. Bibidi decides to reward Buu with sweets when he behaves himself and reseal him in the Magic Ball when he becomes disobedient. Bibidi also uses the Ball to control Buu while he is traveling to another world.

Since Grand Supreme Kai didn't completely take over Buu's personality, Buu's original evil side sometimes gets the better of him, combined with his childish nature. When that happens, Bibidi seals Majin Buu inside the Magic Ball.

Therefore, most of the time, Buu is easy to control. But when his dark persona along with his childish behaviour shows, he disobeys. Bibidi seals him away only then, and also while travelling.

Answer 2

The answer is actually pretty straightforward. At first, Buu became seemingly easier to control. However, after some time he started to become more and more disobedient so Bibidi sealed him up. Here is the quote from dragon ball's wikia page:

It is not until Majin Buu absorbs the Grand Supreme Kai, signaling a personality shift to innocence, that Bibidi is able to take advantage of the monster and he is made to be controlled to a certain extent, serving him cake every night for his cooperation in conquering planets.

However, as Buu tends to become disobedient from time to time, Bibidi devises a Sealed Ball that can be used to temporarily trap the monster. Bibidi decides to reward Buu with sweets when he behaves himself and reseal him in the Magic Ball when he becomes disobedient. Bibidi also uses the Ball to control Buu while he is traveling to another world. Bibidi eventually sends Buu inside his Ball to Earth, which is his next target. However, before Bibidi reaches Earth and releases Buu, he is ultimately killed in battle by Eastern Supreme Kai. Though he effectively lives on through his doppelganger Babidi.

The same thing happened with Babidi, at first Buu was doing everything he was asked to. But after some time he realised he did not really like Babidi and therefore decided to kill him.