The Quarter Inch foot is a mainstay for quilters. This is because piecing a quilt usually requires a 1/4 inch seam allowance. That is, the seam is sewn 0.25 inches in from the raw edge of the fabric. An accurate 1/4 inch seam is necessary for accurate piecing, in which the pieces fit the design precisely. For example,
here is a quilt block:
Here is the back of the block, notice the seam allowance as indicated.
Other sewing techniques use a 5/8 inch seam allowance. Now that I think about it, I'm not sure why it's a quarter inch for quilting. That's what I was taught, and everyone does it that way. It's probably to reduce the bulk of the block.
Anyway...it makes sense that you'd want a presser foot to help you make nice, straight, accurate 1/4 inch seams. Here are my quarter inch feet for my Pfaff:
Above, the upper foot is a plain quarter inch foot. The lower one has a guide (a non-sharp blade) on the right side. You line the raw edge of your fabric up along the blade as you sew.
In the photo above, the arrow on the upper foot shows the slot for the Integrated Dual Feed feature of the Pfaff. Yes, you can disengage Dual Feed, but I find my piecing is more precise while using it.
The arrow on the lower foot points out the guide.
I almost never use the foot without the guide. Try as I may, I just can't sew a straight accurate 1/4 inch seam without the extra help of the guide. It really does make that much difference for me. I ended up with the plain quarter inch foot because it was included in the Quilter's Toolbox accessory pack for the machine.
Notice also that the opening for the needle is a small hole, not a slot. This foot is intended to be used for straight stitch, not zigzag or decorative stitching. It works best with the straight stitch needle plate.