As the soil around your foundation becomes soaked with moisture, it expands and puts pressure on your foundation. Hydrostatic pressure is the constant force of water pressure on the foundation walls. When this force becomes more than the walls can handle, the walls will begin to crack and bow inwards.
This affects the structural integrity of the walls. Water also will then work its way through any opening available, including cracks and openings around pipes or windows.
Water sitting behind foundation walls will find and fill invisible openings in the surface of the porous concrete it touches. This contributes to capillary action, creating a series of tiny veins that will seep on the other side. (Source: JES Foundation Repair)