Holding every other feature fixed, how predicted rent moves as bedroom count rises. This is the non-linearity a straight line could not capture.
Each early bedroom is worth a lot; later ones almost nothing. Predicted rent climbs steeply from a studio (~281k TSH) to a 5-bed (~1.23M), then flattens after roughly seven bedrooms: a 10-bedroom listing is advertised at about the same as an 8-bed. A straight-line model assumes every bedroom adds the same amount; this curve is exactly the diminishing return that let the tree models edge past Ridge.