A walking foot helps move the top layer of fabric at the same pace as the bottom layer. Therefore you don't get wrinkles or streaching in your sewing. If you are piecing two similar fabrics together a walking foot is generally not needed and a 1/4 inch foot goes much faster, but if your fabrics are dissimmilar or slippery than a walking foot will help them move together. Sometimes, when you are sewing long thin strips together it is recommended to use your walking foot to help them stay together and have your strips end up straighter and the same length. A walking foot is of course always recommended when quilting straight lines, as it will help move your layers, top, batting and back together.
sz