Why rest a cake before slicing instead of treating 'out of oven' as finished?

Answer: Crumb firms as it cools

Crumb firms as it cools ✓ — Right: cooling is still part of structure setting. After baking, fats, proteins, and starches continue stabilizing while moisture redistributes through the crumb. That extra time helps the cake resist tearing, gumminess, or collapse under a knife. The useful surprise is that "done baking" is not the same as "finished becoming a cake."

Steam keeps lifting — Steam does not keep lifting the cake after it leaves the oven; the direction is mostly the opposite. Vapor pressure drops during cooling, so the crumb must be strong enough to keep its shape without that push. Resting lets the internal network stabilize while moisture redistributes. Treating steam as a post-bake booster misunderstands why hot cake is fragile.

Crumb dries instantly — A cake does not become dry instantly the moment it leaves the oven. Moisture moves gradually, and some steam can condense back into the crumb while heat evens out. Cutting too soon can release steam and damage a soft structure, but that is different from instant drying. The useful idea is slow stabilization, not sudden desiccation.