Some creatures of magic are borderline humans. It can be extremely difficult to make a distinction between which creatures of magic are actual people or just what their names suggest; creatures. This is especially complicated when it comes to creatures like the Andolians, since they look mostly human, but have decidedly different tendencies and instincts. Or, on the other hand, Confessors are solely human, but have the Confessor's power in them from birth, setting them in a different class from normal people.
It can also be much less obvious than the magic possessed by the Confessors and the Andolians, as in the case of the Mud People. They do not derive any great power or material benefit from their magic, but no other people can call forth the ancestors' spirits through a Gathering as they do. It is a small, simple way that magic affects them that benefits them with wisdom from the departed, and of no use to anyone else.
Others are decidedly inhuman, like the night wisps, dragons, and the tree people. They are creatures existing solely by the caprice of magic, and exist in fragile lands threatened by human encroachment. They are more common in the Midlands, where they are protected by the Mother Confessor, and all but eradicated in the Old World by the Imperial Order's practice of sterilizing the land of the "taint" of magic.
It can also be a simple, innocuous thing that does not seem so important until it ceases to exist, as in the case of the gambit moth, described by Zedd. The waters that flow into the Nareef valley are pristine and clear because they are firstly filtered by the paka plant, which is the only thing that will grow where poison leeches into the water from the soil. The Paka Plant filters out the poison, and in turn produces berries that are encased in shells as hard as steel. The gambit moth lays its eggs on the paka plant, and the caterpillars feed from the paka plant, until they spin cocoons and become moths themselves. The moths have a special dust on their wings that can dissolve the shell-casing of the paka plant's berries. Warfer birds feed on both the gambit moths and the paka plant's berries, and the dust from the gambit moth's wings allows the bird to draw sustenance from the berry, and to spread the seeds in its droppings. Without the magic of all three, all would die out, and the water of the Nareef Valley would be poisonous, spreading untold damage through the Midlands, and possibly beyond. Thus is the importance of the creatures of magic.
In many cases, a creature of magic doesn't actually have any control over their magic, if indeed they have any at all, and it is only a part of their nature that is essential to their dual existence. In a few cases, as in that of the weapons created by wizards in the Great War, creatures such as the Mriswith, the dream walkers, slides, and others that we don't know the names of, they have great power and use it without hesitation.