Timber (both Softwoods and Hardwoods) is a natural product, water content (moisture) changes within timber decking will vary.  The constant changes in moisture result in alternating expansion and contraction, which in turn creates cracks and splits in the surface of the timber.

When the surface cracks and splits it exposes the inside of the deck boards which are generally not treated (this varies according to timber species / mix of sapwood to heartwood, the treatment cycle used and chemical treatment system).  Micro organisms will then attack the timber, reducing its life span. (Treatment that should be applied each year to slow down this decay process.)

Other problems associated with timber decking are that the cracking can create dangerous splinters and sharp edges that can cut bare feet eventually, the visual appearance of the deck will generally become unsightly and can devalue the attached property.