Your brain is the user interface for your body.
Your soul is you. It is where you think, where you feel, where YOU most truly and actually exist... but a soul can not control a physical body directly. Souls are mostly incompatible with the physical world; so, you need an interface. Something that can translate between an ethereal input/output interface and the physical world. And that is where your brain comes in.
When your eye sees, your brain takes in the bio-chemical signals of your nervous system and creates an ethereal pattern that a soul can perceive the same way a computer needs a video driver to convert software data into a signal that a monitor can output. And when you want to move, there are ethereal patterns that your soul manipulates to trigger electrochemical patterns in your brain much like how your user interface can only trigger the available functions in a videogame. If you think of it like this, when you play a videogame, you are not writing out machine code processes for every foot step and motion, those are all scripted behaviors inside the simulation. If something happens to this interface, then you become unable to do what you want to do in the simulation.
So when you suffer a traumatic brain injury, your soul still knows what's going on, but his interface has become unresponsive and glitchy. When it tries to bring up dialogue options, half the options aren't there and when you do pick one, the wires might get crossed and your body does something else instead. And when you die, it's like a total system crash. The game stops loading, you can't log in anymore, and so there is nothing for a soul to do but just get up and go on with its exitance (or create a new character if your setting allows for reincarnation).
Imagine the following 3 dialogue boxes in a videogame. A whole and intelligent spiritual being playing a baby may itself be smart enough to go work at Apple as thier lead developer, but the only options the brain allows for are various forms of gibberish. For a grown man, the brain has developed, and the soul has "leveled up" his character enough to unlock all sorts of useful (and many ill advised) options to work with. Then you have the elder or disabled self where the brain has become compromised, so your UI is compromised.