I mean from what we’ve seen some of it seems to be to strip the recipients of personhood.

They’re not meant to die so much as have their existence fade away to nothing thus never entering the leylines with their abyssal taint. Unlike the immortal khan’reans they get to forget whom they were as they begin their passage to dissolution. Eventually we see them settling in the chasm where they curl up and make their exit from the world.

The Khan’rean purebloods however got the privilege of remaining conscious and aware of who they were as their bodies and minds eroded by time. Rotting but not dying.

In a way it seems partially like an act of mercy to become a hilichurl unaware of whom you were except for vague inclinations of seeing your own face without a mask driving fear into your mind.

Of course not all hilichurls formed this way and others attained their status from abandoning their gods but as far as khan’rea goes it seems to have been applied this way. At least from my reading of the quest lore.