Architecture is often presented in plan — the horizontal cut through a building — but the section (vertical cut) reveals what plan cannot: how spaces stack, how light moves through floors, the relationship between interior height and human scale.
Le Corbusier's section drawings show this: the free plan only makes sense with the section. The pilotis (columns) free the ground level while the roof garden reclaims it, but you only see this in section.
The section is also where structural and spatial decisions are most directly in tension. The beam that spans 12 meters eats into floor-to-ceiling height. Good architecture resolves these tensions in section before committing to structure.