Trees reduce the chances of rain harming soil. The roots of trees create multiple channels that can hold soil in place. The foliage atop trees prevents harsh winds and rains from attacking soil.
The roots from trees and other plants stabilize the soil and hold it in place. Vegetation also lessens the impact which precipitation hits the ground and shelters the soil from wind erosion as well.
While smaller precipitation events may not greatly affect the soil, large events where the soil cannot absorb the precipitation quickly enough results in soil being washed away. Bare soil is far more likely to be negatively affected than soil that is sheltered by plants and held in place by their roots.
The same is true for wind. Bare soil is much more likely to shift and be transported than soil that is held in place by plant roots and sheltered.