Use the Weathering slider to add subtle wear-and-tear to edges and flat surfaces. This adds "dirt" to corners and slight aging to materials like wood and stone.
This effect enhances the bounce of light, making interiors look significantly more natural. For the best results, set the amount to around 30-50% to avoid over-brightening the scene.
For architectural shots, always enable this to ensure your vertical lines remain perfectly straight.
Flat, perfect surfaces are a "dead giveaway" of a digital render. Real-world objects have imperfections.