There are countless things that are true that aren’t useful. Or, the uselessness of a thing does not affect its truth.
Regarding the intellect, like the will, it’s an aspect of the soul, or “form,” of the body, in the Aristotelean sense. That is, the soul is the organizing principle of the material body. It is a simple spiritual substance —it has no parts.
In life, the soul is united to the body, causing the body to act coherently as an organism. When the soul is separated from the body at death, the body breaks down into its constitutive components —it decomposes.
The difference in bodily functions in the moments before and after death are incrementally different —no different in degree from the incremental bodily changes leading up to death. Yet something radical happens at death. The body ceases to act as a whole —even while components of the body may continue to live for a time.
If the soul is the organizing principle of the body, it is fully integrated with the body, and changes to the body will affect the soul (while not decomposing it), and changes to the soul will affect the body.
Finally, the idea of the soul accounts for the experience of self, which atomistic materialism cannot explain in a noncontradictory manner.
Is the person born with dwarfism because the organizing principle of their material body is a defective “soul” - or is it that the organizing principle of their material body is DNA with a defective bone elongation factor?
Certainly in our past that sort of superstitious and primitive thinking prevailed - and anyone born with dwarfism was reviled as having a stunted soul.
Does someone born with Down syndrome has less of a soul than most people? They are certainly less intelligent than most people. Do they also therefore have less of a soul, or do they have more DNA than was optimal?