Google's Material Design introduced elevation as a first-class design primitive, assigning specific shadow values to specific component types (a card at 2dp, a dialog at 24dp, a tooltip at 8dp). The logic was rigorous: elevation communicated the relative importance of components and the order in which they demanded interaction. A dialog at high elevation blocks access to everything below it — elevation as attention control.
Elevation without a consistent light model becomes arbitrary. Material Design specified that light came from directly above, casting shadows downward. This consistency meant that shadows were predictable: a surface 2dp high cast a small, tight shadow; a surface 24dp high cast a larger, softer one. The system had internal physics.
In dark-mode design, shadows become less effective (they disappear against dark backgrounds) and elevation is more effectively communicated through surface brightness. This is the principle used in Material You's dark-mode elevation system: surfaces higher in the stack get slightly brighter, using light as a metaphor for closeness rather than shadow as a metaphor for height.