The (total) magnitude is just a convenient scale to measure of how much light that is radiated/reflected on an object. If the light was radiated equally in all directions, the light (all photons) would after a time have travelled equally far, defining a sphere. Since the photons travel further away every second, the surface area on the sphere increases as r^2 (r=distance to object), but the number of photons is the same. That means that the light density, or the number of photons that hit our eyes/cameras, will decrease with the same factor, r^2. That's why they created the apparent magnitude. The (total) magnitude is a measure of how much light that leaves the object, while the apparent magnitude tells us how much light will hit the earth, or equivalently: how bright will this object appear on the earth.