A real world application of piecewise functions are in designing profiles and shapes for cars, airplanes, and other such devices, one usually uses pieces of Bézier or B-spline curves (or surfaces) during the modeling process, for subsequent machining. In fact, the continuity/smoothness conditions for such curves (usually continuity up to the second derivative) are important here, since during machining, an abrupt change in the curvature can cause the material for the modeling, the mill, or both, to crack. In addition,  for piecewise functions the classic example is friction---say for a block sitting on a rough horizontal plane and subject to a continuously increasing horizontal force. The frictional force rises steadily to match the applied force, and then it drops back a bit to a constant value when the block begins to slide.