Using continuation of an element to create harmony and unity in a piece many times uses line as that element. Drawing a line through various elements on a page as shown here in cut paper helps to unify the elements and draw the eye from place to place in the design.
Some styles tend to use continuity as part of the style itself. Art Nouveau and Celtic designs have flowing, curving lines that tend to curve back on themselves. This gives a sense of harmony in the overall design.
One way to use line in a piece to create harmony and unity is through quilting or free machine stitching over the entire piece. Many times, this draws diverse elements together and puts the finishing touch to unify the piece.