1. What is a soul?
2. Who (and/or what) has a soul?
3. Why would a soul want a body?
4. What does a soul actually do?
5. If the soul is "immaterial", how does it communicate with the material brain?
6. If you had no soul, how would you be different?
By ancient human assumption, souls don't lie in our realm. In modern science, this advocate means souls don't lie in our spacetime. While our science doesn't have a concept on how things possibly work in another spacetime as science is experiment-based and we can't go to such a spacetime to establish experiments. So all left is all about philosophical speculations.
That said. In Christianity or Judaism soul represents one's true identity. Speculation is that soul is needed because it represents the real you independent of your body. Humans need a body as our image for us to recognize each other. It is so because humans are designed not only to live in a physical world but also that their lives are staged, say, the earthly stage and heavenly stage and with our body dies and decays in the first stage (at least). So the only thing which can continue to represent the real you (along with the decay of your old body) permanently is your soul.
Your spirit on the other hand is used to store stage-dependent information. For example, your earthly affections are stored in your spirit such that on earth you have the affections for your parents, children, soulmate, and so on. These affections will be gone along with your physical death. It is thus said that your spirit will return to God on your death.
The soul is also closely tied to the Final Judgment. It is to make sure that it is "you" who is under the judgment in court instead of "a clone of you". If only your body is resurrected, you can argue that the resurrected body is not you but a clone of you and thus should not be judged. However in the presence of a soul, as reckoned by the witnesses (chosen saints and angels), you are the "you" who should stand the trial.