The nature of incident reading is that it leaves aside the reflectance of the subject. In the same light, with the same incident reading, a middle grey subject will be rendered middle grey, a bright subject will be rendered bright, and a dark subject will be rendered dark.
This happens "automatically" by the sheer fact that the bright object reflects more light than the middle grey objects and thus renders brighter on the slide than the middle grey object, given the same exposure. It works very well until the "whiteness" or "blackness" of the subject is within the dynamic range of the film.
With reflective reading the photographer must be aware of how different the subject is from middle grey and adjust the exposure based on that, thus "placing" the subject in the film curve. With incident metering the photographer doesn't need to do the thinking. The subject will be "placed" in the film curve by its own reflective power "automagically".