Not only is it a 1/4" but it's usually a 'scant' 1/4".........I don't really know the answer, but it might have to do with all the fabric when nesting and connecting corners and centers. You can make it any size you want if you're making a quilt for yourself but if it's a block for a swap, they should all be made using the same directions. If you add up that extra 1/4" per seam, it could add quite a bit to the cost, also.

Pressing the seam to the side stop bearding. (That's were the parts of the batting slips up through openings.)

Yes, pressing to one side used to be to prevent bearding, but I think that was back in the "old days" when quilts were mostly hand pieced. Sewing machine stitching doesn't leave the space like a hand running stitch does. And nowadays battings are made differently, many having at least one side bonded with something similar to interfacing.

I don't know where the 1/4" seam originated. Clothing patterns typically call for a 5/8" seam allowance.

Donna

I remember my grandmother saying she made her seams small so she could easily keep them even. Her's were not always 1/4 inch, but they were always quite narrow. She didn't always press hers (no electric irons back then) but they did go to one side. The "batting" they used was cotton they grew, cleaned, and spread between the layers of fabric. Turning the seam to the side prevented the cotton from bearding at the seams. The layers were then quilted to keep the cotton from shifting too much within the quilt.

Pressing a seam to one side is to also strengthen the seam for quilting in the ditch. If you press your seam open, then your ditch quilting is going over just the stitches/threads. If you press to one side, then your quilting goes over those same stitches but also through a layer of fabric...thus it's stronger.

Pressing seams open would lessen the number of fabric layers you'd go through if you were hand quilting on both sides of the seam. But pressing to one side would lessen the layers on only one side of the seam.