This magnetic tool is great for controlling that you are sewing parallel to your edge. Simply place it on your sewing machine/serger/cover stitch machine and follow along. It provides a good length so that you can position the fabric and feed straight towards the needle.
We have learned that one needs to test how strong the magnet is, as some seam guides we tested were too strong and they messed with the mechanics of our sewing machine. This one is fine.