Originally, as a domineering CEO, Sargeras could do whatever he wanted without needing to explain to anyone. If he wanted to add a few more commanders, he didn't need to explain to Roy at all.
But how should it be put? Roy still had some status in his mind, especially Roy's strange power that could bring him to enter and exit the Void freely. Therefore, even though he wanted to add more commanders, he felt that Roy's position as the first commander was very stable, so he couldn't help mentioning it to him.
But how could Roy care about this? He had time jumped here from the future, so he naturally knew the power structure of the Burning Legion in the future. He, the original first commander, would disappear inexplicably for hundreds or even thousands of years from time to time, so he was gradually marginalized. He himself wasn't that concerned about the Burning Legion, so he felt that Sargeras could do whatever he wanted.
