In the Grimoire-verse, Souls are not some irreplaceable thing that—once lost or destroyed—can never be regained. A soul is simply a spiritual reflection of a living body, and will emerge within a few days given the opportunity.
The presence of a soul provides a barrier against possession by hostile spirit beings, who will otherwise have free reign to do as they please. Those who lose their souls by any means will therefore seek other protections until a new one can emerge, lest they are overtaken by spirits or killed without prospect of an afterlife.
While many spirit beings are independent entities, and most souls will naturally pass through to the afterlife, a few are doomed to spend their existence wandering the mortal realm. Some become simple ghosts, whose attachment to things in life has anchored them. Others may become twisted abominations, such as the Wraith, which will appear when a soul is freed from a living body after having been drained of spiritual energy. Such a spirit is warped by the experience, and lives with a constant hunger to replenish that energy by drawing it from others.
A soul can also provide a means of resurrecting the dead. By drawing back the soul, it can be transmuted into a physical body that will shortly produce its own soul. The subject of such a resurrection should do everything they can to avoid the transmutation being undone, however, as they will end up with two identical souls and no body at all.