After the disappearance of the god of Death, Ymigard, souls seem to not travel on after death.
If not dealt with, the loose souls will degrade and warp, and turn into ghosts, or worse.
In most cultures it is customary to take some responsibility for what should be done with your soul after death, it is seen in many cultures as a sign of becoming an adult. It is “polite” to carry one spare, empty soul jar for your own soul. And most travelling parties contain one acolyte or spell caster, that can handle the soul and move it into the soul jar. Further plans for your own soul, like moving it to a prepaid repository, or delivered to your family, is usually noted on a piece of parchment inside your spare soul jar.
A soul in a soul jar will degrade and warp, but much more slowly than if it is “loose”.
In addition there are measures that can be taken to remove wayward souls after the fact, but moving and directing “fresh spirits”, is easier and can be done by most spellcasters.
The cheapest option is to donate souls to magic schools and similar. An other option is to deliver them to storage, which try to provide some semblance of a different life for the souls, through magical means. Some times there are ways to bind the souls to matter, which may give them a “second life”, like becoming a Bound or an Oozecut. “Giving the soul something to do”, is the best way to keep it from warping. It will still degrade over time, meaning even Oozecut and Bound will eventually “die of old age”, were the soul has degraded to such a point that there is no soul left to speak of.