Our soil in Western Australia is very sandy and hydrophobic in nature (in most parts) this can lead to water retention problems that sometimes only manifest themselves in summer. Some soils may repel water off their surface while others may not hold onto the water. This can lead to soils drying out quickly or just running off the surface of the lawn/garden bed instead of penetrating the soil. This generally results in lawn and gardens struggling due to lack of water even though you may apply more than enough water. Over watering does not help alleviate this problem. Applying wetting agents however helps the molecules of water stick to the soil particles instead of to other water molecules thereby "wetting" the soil.
Wetting agents can be applied at any time of the year but generally the best time to apply a wetting agent is in Spring when the soil is still moist from rain. It is much harder to wet a soil that is very dry.