The wider sidewall raises the edge knobs without raising the center knobs, which makes a less round or more "square" tread cross-section profile.
Also the tire casing tension under the edge knobs is increased to be firmer, increasing the tire's stability, and allows lower tire pressure when desired for less rolling resistance over rough terrain from increased bump compliance, and a larger tire patch when weighting the tires while braking and cornering.
Cornering traction is much improved on dusty hardpack to very loose gravelly dirt or loamy covered with leaves, as a result of this combination of flatter tread cross section, stiffer edge knobs, more knobs in contact, and lower air pressure, all enabled with wide rims.
Changing direction has quicker response with the more stable and firmer structural tire edge with wide rims, without feeling harsh and choppy, and no more sudden washing-out of the front wheel, or as easily skidding the rear tire while cornering.
The whole bike becomes more stable on wide rims, feeling like a longer wheelbase bike in directional control but with quicker more positive turn response for more nimble handling and corning grip. Holding a line over loose round rocks though "rock gardens" becomes much easier, without getting kicked side to side when not centered over each loose rock.
The result of going to wide rims with the same tires is immediate increase in confidence while riding loose dusty, gravely, rocky, and wet conditions; enabling higher speeds, harder braking, and more precise and predictable cornering.
