I remember during my first quilting class I automatically pressed the seams apart from the inside.
Garments have 5/8" seam allowance because we put pressure on the seams when we move, and a smaller seam allowance would be more likely to ravel and come undone. Quilts have the smaller seam allowance because there is not the same pressure on the seam, and a larger seam allowance would make the seams bulkier and the quilt heavier.
Grain is very important when sewing garments, but not as important with patchwork.
And the big one - a sewn garment may or may not fit me when it's done. Quilts always fit!