It is a foundation in order to use small strips that would otherwise not be easy to sew together. You then usually cut to the size of the base. In the olden days newspaper was used but mostly a foundation fabric. The foundation could be cut in a number of shapes. I have some old blocks that the foundation was a diamond then sewed together to make stars. It was and stil is a "use up"method of making use of otherwise unusual sizes of fabric. irregular width of strips could be used to make a new fabric.