I think cremation is also done to prevent the spread of plagues? I remember it vividly in the End Times, but I'm pretty sure cremating bodies for plague victims is standard practice so the plague doesn't spread.
For some plagues, it's believed that the souls of those killed by them become Nurgle's property, so there's no more damage to be done by cremation. This is explicitly the case for Nurgle's Rot, and the souls of all that die from it become the demons known as Plaguebearers.
I am curious about the Amethyst College take on this. Have they tried to conduct empirical research to more precisely determine what happens after death? I can understand that this would be intensely controversial given the religious connotations and connection to necromancy but I would think that the potential benefits would still inspire some serious effort.
Upon death, the soul goes deeper into the Aethyr. Amethyst Wizards can see this happen. To research beyond this point would require physically travelling into the Aethyr to follow it in its journey. Nobody who has tried this has returned to publish a paper on it.