Uh... Rephrase of question - what does the lifeforce/soul actually do in a body that is necessary for something to function properly (and which is not already done by, say, adenosine triphosphate [one of the main energy carriers/very short-term storagers in cells])? What role does it fill aside of being something that can carry on to afterlife and provide magic? Why would underabundancy of it be harmful in the first place? How does the lessened amount of it damage the body?

(Tired?)