If I remember correctly, exorcists like Muryeong and Muheun can only seal spirits. The only way to completely get rid of a spirit is to kill it which very few can do but it’s also forbidden to do so bc smth smth forgot. Seungjoo not only gets rid of the spirits for good without killing it via purification but he can also peak into its consciousness. He needs to look into each spirits memories to find proof of the exorcists directors actions so they can kick him out for the court case.
Slight spoiler for Muryeong story but...
Muryeong literally purified a spirit in the newest chapter.
Ohhhh I understand your question now. I’ll explain it down below. (Spoilers)
In muryeong’s story on the latest chapter muryeong implies that despite purifying an evil spirit not being possible, he and very few (or maybe only him) people can do it, though it seems there are repercussions to doing this. The fact that both muryeong and seungjoo can do such a rare thing is most likely connected to the story considering that they’re born in the same year, though we probably haven’t reached that part in the story yet. So in this universe; the only known people to have purified spirits are muryeong and seungjoo which answers your question on why Muheun can’t purify the spirit himself. And I repeat myself again, seungjoo specifically can peak into a spirits memories which they need to provide evidence for the court case.
It seems that muryeong and seungjoo’s abilities are different as well, as muryeong’s ability returns an evil spirit (a dead human soul) to its original state while seungjoo purifies a different type of evil spirit (accumulations of grudges of different concepts such as the grudge of a forest for being burnt down.)
All in all, both stories are still ongoing so hopefully we’ll have a clearer answer to our question by the end of both!
This is a question I've had for a long time, but don't Seungju and Muryeong have the same power of purification? And if so, why don't we just have Muryeong do this, as I cant imagine Seungju is nearly as proficient as someone who has been doing it their whole life.