Water molecules are polar and thus strongly attracted to each (versus the surface that they are on). When you apply something like an oil or wax, which is not polar and thus not attracted to water, this creates the surface tension and repels the water. With waxes and sealants you are increasing surface tension and you get lots of small, tightly defined beads. As that degrades, the surface tension goes down and the beads get bigger. A coating is an entirely different story - you are actually purposely decreasing surface tension down to the point where the surface is slick enough that water just slides off, aka sheeting.
