Values are lights and darks. Light reveals form by lighting areas and leaving areas in the dark. Plane changes result in abrupt value change. On a sphere the change is gradual.
Relative value need not be extreme. THe change can be very subtle. Think about a rock light on misty day. The sides may be only slightly darker than the top which is not as lit or in shadow. Play with how gradual a value shift you can make!
Giants’ Legos