The seam guide is a marvellous little gadget. I wish I had known about them earlier. My mum never had one, and they were unheard of at school, so I never tried one out until I started acquiring old sewing machines and finding extras in the compartments or drawers.
Now I'm totally hooked. If you are doing patchwork and need to a quarter inch seam allowance, the seam guide is ideal. It is a lot more reliable than the method I used before, which was to stick a piece of white tape on the bed of the machine and run the material along the edge of the tape. The seam turned out fine, but getting the adhesive wiped off the machine afterwards was a bit of a bore.
The machine is a 1937 (more or less) Frister and Rossmann transverse shuttle. It sews beautifully and weighs a ton. I love it.