It gives you a hard edge to move the fabric against as a guide. It's magnetic and you can move it around to various seam allowances. It works best on a machine that has a large metal plate area. There are some machines that have tiny plates...painters tape or masking tape might still be a good idea for those types of machines. It really has improved the kid's sewing by using this guide! Give it a try if you struggle with consistent seam allowances.