Soil wetting agents are designed to overcome water repellence in soils. Sandy soils, soils high in organic matter and potting mixes are the most likely to become hydrophobic. When these types of soils are watered with a hose, the water simply rolls off and is not absorbed. This can be a trap for gardeners who think they have watered their plants when in fact the water has simply rolled to the sides of the pot and out the drainage holes without wetting the soil at all. So "well-watered" pots can often be seriously drought stressed.
Soil wetters help to overcome the effects of waxy organic coatings on the surface of the soil and the surface of organic matter so allowing the water to penetrate and be absorbed.
Soil wetters are essentially the same as detergents. They reduce the surface tension of the water and allow it to wet the waxy surface of the soil particles allowing water to move into the soil through the pores. Soil wetting agents belong to a class of chemicals called surfactants. There are different kinds of wetting agents/ surfactants and they are used for different purposes. In horticulture, as well as soil wetting, surfactants are used to allow various products such as herbicides, fungicides and fertilizers to spread out over the leaf blade of plants and be more readily absorbed by the leaf tissue. These "spreaders" are designed to work quickly but do not last long.