A polygon is a closed figure bounded by line segments. While common sense tells us that the figure can be broken into individual lines, each being clipped individually, in certain applications, this method does not work. Look at the following example.
A solid arrow is being displayed. Suppose the screen edge is as shown by dotted lines. After clipping, the polygon becomes opened out at the points A and B. But to ensure that the look of solidly is retained, we should close the polygon along the line A-B. This is possible only if we consider the arrow as a polygon – not as several individual lines.