Capillary action. In simple terms, water really likes to wet certain surfaces (e.g. wood fibers), and is willing to do so against a small amount of gravity. If you have a lot of surface area per volume of water, then the water can be pulled up quite a distance.
Okay, So why does it matter?
Your home contains a lot of wood and most of it is covered, but what about the exterior? Paint and pressure treatment can only go so far. Well, this is important because your homes roof is by far the larges surface area of the home that comes in contact with the most amount of moisture.
Without the correct space between your shingles, siding and trim, Capillary action will occur. The Siding and trim in contact with the roof will wick the moisture and deteriorate long before other parts of your home.