Surface properties can be used in various ways. For example, often a quick check to see if a native oxide is removed from a silicon wafer is to run di water over it. Typically (unless specifically treated), di water will bead as it runs across the pure silicon (hydroboic nature) while any silicon oxide would cause the water to hug the surface (hydrophillic nature).  The contact angle can be used as a good indicator for if a surface is actually clean.