The one on the left fits a machine you can screw it so the fabric runs along the "hump" adge to sew a straight seam. You can move the piece side to side depending on how wide you want the seam. I guess you could use it for decorative stitching too to keep lines straight. I do not know what kind of machine it is for, but guessing a White, Singer, Kenmore, maybe an old Montogemery Wards machine. These are machines I had experince with in the 50's-60's. Take care.
Linda