Since matter exists for the sake of form and not vice versa, we must discover, on the side of the soul, the reason why the body should be united to it. Hence it is said, in the De anima, that the soul is not only the form and mover of the body but also its end. Moreover, it is evident, from the preceding Disputed Questions, that it is natural for the human soul to be united to the body. For, although the soul is lowest in the order of intellectual substances [he means in comparison to the rest of created intellectual substances, all of which are angels - RdP] (as primary matter is lowest in the order of sensible things), it does not have intelligible species naturally impressed upon it, as superior intellectual substances have; but it is in potency to them because it is like a wax tablet on which nothing is written, as is said in the De anima. For this reason it must receive its intelligible species from external things through its sensory powers, which cannot perform their proper operations without bodily organs. Consequently it is necessary for the human soul to be united to the body (Disputed Question on the Soul 8, in Bourke, The Pocket Aquinas, p. 135f.).Unlike the angels, we do not have our knowledge by nature. We are not born with it. Consequently we must acquire it. And the means we have for doing so is our sensory powers, which work through our bodies. Hence we need our bodies in order to acquire knowledge. They are the only means we have available to us.
Of course there are those who are skeptics and who challenge the reliability of the senses. That's all fine and good in the ivory tower, but the simple fact is that even these skeptics still eat and drink and breathe and walk about. They read arguments and write them. And they could do none of these things without relying completely upon the sensory input that they receive from their bodies. When the rubber hits the road, we are all empiricists.