I think the reason that the Other kingdom wants Sanghee is because they know about her previous life. After all they wiped Jinsu's memories of his previous life and brainwashed him so he would do their bidding. So I don't know what they are planning to do with her and Jinsu when they get both of them under their power, but I think it should have something to do with their reincarnation. Perhaps those who are reincarnated have innate super powers that are crazy even in comparison to Sanghee's father.
I feel like another big part of it too is that while Koryo knows a bit about her power (not the full story, but that it exists) they prefer her as an actual human being. This juxtaposes the emporer who sees people for what powers and magic they have as opposed to who they are as human beings (think of freesia... sure she was powerful, but once she began to threaten him he threw her out, sees jinsu only as a source of power. Etc etc) so this isn’t to say that koryo DOESNT want sanghee’s power, but rather they see her as more than just her abilities and want her around because of that!!
I dont get it. Why does the other kingdom value the MC's power but her very own kingdom doesnt?