These two items can be lifesavers if you’re sewing slippery fabrics, sheers, or fabrics of a loose weave, both of which run the risk of getting pulled down into the guts of your bobbin case when sewn. Your fabric doesn’t want to explore the depths of your sewing machine, nor does your sewing machine want it to and will likely protest the situation most vigorously by throwing up its timing (aka: the synchronized movement between needle and bobbin) and requiring an expensive trip to the shop.
The smaller hole in the sewing plate and foot reduces the chance of fabric being pushed/pulled down inside your machine and thus reduces the chance of it getting snarled, puckered and otherwise breaking your heart. Snarled and puckered seams are just what you don’t want when sewing sheers.
Depending on your machine, the throat plate will cost $15 – $40, and the foot between $10 and $20. Like everything else related to a sewing machine, there is no one-size-fits-all-machines, so if you’re not sure what to get, visit your local sewing shop with the details of your machine’s make and model and they’ll set you up.
When using these parts, you can ONLY use the straight stitch setting on your machine. Forget what you’re doing and start on a buttonhole and you’ll have a broken needle faster than you can say flying fragments of sharpened steel.