The thing a spot meter does very well, for those who are experienced with it, is tie a specific point in the scene to a specific point on the characteristic curve.
It is a truly useful tool and that tie is a good way of thinking, but it is also subjective unless you know what to expect from your target.
Bill's hand, a grey card, new jeans, faded jeans, your camera bag; any target that you know the offset to will work fine. Used with a known target, by a skilled person, spot meters are every bit as objective as incident meters are.
Incident meters are really good at measuring the light that is falling on a subject/scene. If we know how much light is falling on the scene we can know exactly how the subjects we see are going fall on the film. The offset works exactly the same way as if we had put known target at the same point.
What neither a reflective meter or an incident meter know is what they just took a reading of.