Most pokemon, whether living or dead, can claim a single soul; a few, such as Dodrio, claim three and are therefore often riven with conflict. And then there are the Spiritomb, where 108 souls joined together somehow work seamlessly together to cover each others' weaknesses and become a remarkably powerful pokemon.
The reason these souls act as such effective allies is still debated. Some have suggested that, because there are so many, each soul realizes the futility of trying to control the pokemon's body on their own and they make every decision by majority vote. Others suggest they take turns, and each spirit fears losing its turn if it rebels. A few suppose that the souls, having lost the ability to control their body, have made peace with their form and spend their time losing themselves in mental conversations with one another and forming powerful friendships. Others say that Spiritomb souls so hate being sealed together that they are united in vengeance upon the world which sought to punish them for their sins. Asking the Spiritomb themselves has resolved nothing, for each one gives a different answer. Perhaps which reason is true differs based on the individual Spiritomb, but all have been bound together long enough in their keystones that they have found a way to collaborate.
No matter their own rationale, Spiritomb have become a powerful symbol of unity, used by many governments to symbolize the common people working together for a myriad of goals, and not only because of the souls sealed within each of these pokemon. For while a single or three-souled pokemon can be caught by a single trainer, only a powerful individual who can count many pokemon trainers as their allies can release a Spiritomb from their hellish keystone prison and make them their own.