Like the old saying goes, some of the worst deeds are done with the best of intentions. Oka didn't spend a single second to consider how the reincarnated students would feel about being forcefully relocated into the elven domain, where the local population looks down on them and doesn't want anything to do with them. So, they are left to toil in the fields to grow their own food, I assume, instead of adventuring out there in the fantasy world. They are nothing more than prisoners.
It's one of the scheming parties in the world, opposing the other ones, such as the church. I guess they are nominally friendly with some human nations but not with others. I doubt the place has other strategic importance. The attackers just want to remove a competitor. Considering how sneakily the elves generally behave, it's no wonder.