USD versus conventional forks
Suspension-wise, given springs and dampers of equal quality, there is no difference between upside-down and conventional forks.
But the fork leg is not a suspension unit, it is also a structural chassis member. This is where the USD unit has the advantage.
When in a turn, or more significantly on the brakes, there is a bending force exerted on the fork leg which is concentrated at the lower edge of the bottom yoke.
At this point the greater diameter of the USD leg makes it stiffer, keeping the wheels in track.