Most of our sandy soils become water repellent as the weather warms up and the soil dries out. On a sloping site water may simply run off instead of soaking in. On a level site the water will pool and only soak in a few small areas leaving most of the soil dry. This is a very common problem in lawns where it leads to dry patches developing. Many people react to dry patches in their lawn, by applying extra water which, while it may eventually overcome the problem, also wastes a lot of water.
A far better solution is to apply a soil wetting agent which will allow the water to soak in where it falls.