It's often used as the top thread when doing trapunto - not sure if it's used in the bobbin as well. The design is stitched out on the quilt with batting beneath first, which is then cut away. After the whole thing is quilted for real with another layer of batting and proper thread the washaway thread on top is washed away, and the cotton from the bobbin stays in the quilt.
Fancy making a whole-cloth quilt?