To Dark Sect disciples, the Dark Realm was like a world completely under their control. Like how the heaven and earth were divided into yin and yang, Dark Sect disciples in the Dark Realm could do more than herd souls, they could cultivate.
And the special Dark Qi within the Dark Realm was an extremely nourishing item equivalent to Spirit Qi for the Dark Sect. It could cause their cultivation to incorporate both yin and yang and far exceed that of other sects.
For Dark Sect disciples back in the day, everyone had a fixed right to enter the Dark Realm to cultivate. But there were still cultivation level requirements. They had to be at least at the Planet realm in order to enter. So, Wang Baole had heard of and knew about it while in the Dark Dream, but he had never entered it before.