People have asked me this question a few times now, and as I am wont to do, I will explain why there is both magic and medicine simultaneously in Storm.
The medical practice in Storm has been around for centuries and is quite advanced and capable. However it can't do more than the human body will allow as is the case with any medicine it must work within limits. What magic does in this regard is simple, it accelerates and aids in the body's processes; hastening and augmenting the healing in a way that medicine cannot do. Now in Storm there are things that magic cannot do, such as reattach absent limbs or fix broken bones. If you were to cast a healing spell without first splinting the bone in the proper position you'd end up with an improperly healed bone. The same applies for missing limbs, if the wound has already been closed and healed over there is nothing that magic can do, at least not properly; in addition if you were to try and reattach a limb without first placing it back properly over the wound (usually with stitches and splinting) the limb could be reattached in an odd manner. That's why in Storm you see both doctors and healers working in the medical corps and why those in the medical profession, whether mystically or scientifically trained are often cross trained to act as both a medical and magical doctor so they can properly administer aid.
A good example of what I'm talking about would be this, in the KH and FF games, you get hurt you just cast Cure or Heal and voila, you're good to go. In Storm that would work on minor things like cuts, bruises, or sprains but on something major like a severed limb, broken bone, or massive epidermal trauma you would be asking for trouble just casting a random spell on it. It might even kill you depending on the kind of wound you were dealing with.
Potions take the middle ground in that they are not truly magical, they accelerate the healing process, restore vitality (stamina, fatigue etc) and dull pain; but this isn't because of any magical properties, it is because the plants and the like that are used to make the potions have these curative properties and by combining various ingredients you come up with viable first aid. This is why the typical procedure in Storm medically is to give a potion to a patient first thing before administering medical or magical aid. The potion serves to calm the victim, ease their pain and strengthen their body's natural defenses but not to the degree that a healing spell could ever do.
So that's the gist of it, I hope that explains away any confusion for my readers.