Similar Shapes Associate
The circles in this illustration, because they are all the same shape, form a bond with each other. They seem to form a V shape.
Similarly the squares form an upside down V.
What at first is just a collection of circles and squares takes on larger structures because our minds associate similar things with each other.
The result is to give a certain amount of unity to the design.
If you find a quilt getting cluttered or confusing, you might want to try giving it structure and unity by using similar shapes to pull it together.