This is when I have the short pieces of sashing already attached to the blocks, and am attaching the sashing to the long rows of blocks and sashing. If I don't use the walking foot, the the short pieces of sashing don't fall in line with the other side and it looks terrible. So I use the walking foot. Getting my 1/4 in seam is a challenge cause its hard to judge using the walking foot. But it beats having my sashing falling in the wrong spot.