The Ambient occlusion section darkens corners, edges, gaps and the spots where one piece touches another. It is what gives the model weight: without it the pieces look loose; with it, they rest on the ground.

Controls

Enable ambient occlusion: Turns the effect on.
Distance: From 0.1 to 10 meters. A small value marks only the contact points; a large value darkens whole rooms.
Intensity: From 0 to 2. The strength of the darkening.

How to use it

Set the distance by the size of what is open: half a meter for a tabletop object, several meters for an interior.

The effect shows a little grain in dark areas. If it bothers you, use global illumination, which replaces this section while it is on.