An incident meter tells us the exposure to use for an average scene's range of tones, based only on the intensity of light falling on that scene. Where it falls down is that the incident meter has no idea that there may be certain parts of the scene which are not capturable because the tonal range of our medium (e.g. color transparency) is exceeded by certain parts of the scene.
In the case of using the Zone System, we might decide to bias the exposure in the direction of the part of the scene that falls off the capturable range, simply because our scene might not have the amount of tonal detail in the shadows, so we tweak our exposure and adjust in the processing.
The incident meter cannot tell us when a studio shot exceeds the tonal reproduction capability of the offset press, whereas a spot meter can identify that issue for us, allowing us to adjust our lighting to bring the entire range of tones to within the range of the offset press making that photo on the printed page.

These are the reasons that I own both an incident meter and also a one-degree spotmeter.