Vegetation allows for greater infiltration by anchoring soil and keeping it from drying into a hard barren surface through protection from direct sun rays. When the soil is soft, infiltration is greatly enhanced.
Living and decaying roots create a network of well connected channels in the soil called macro pores. Flow through these macro pores can be several hundred times faster than through the soil matrix.
Leaf litter produces humus. Soil structure is improved as soil particles are cemented together by humus and sugars excreted from roots. The improved soil structure permits increased infiltration.