Welting or sometimes called piping requires a strip of material around a welt cord that varies in thickness, normally from 4/32 to 6/32, where as used between 2 other pieces of material when they are sewn together. Upholstery fabrics and leathers are sewn together using a thicker/heavier thread compared to garments or bedding. Also requires a industrial walking-foot sewing machine that produces a longer stitch length. With this thicker thread and longer stitch length allows the edge of the seam to open up or pull apart to a degree when tension is applied across the seam. The welting adds 2 more layers of fabric to the seam creating a tighter seam and also hides any separation that the seam may have.
So its not only their for looks.
Without the welting sewn into any upholstery seam the seam needs to be re-enforced. Or it will most likely fail.
A single seam should be folded back over on itself and top stitched (similar to the stitching of the legs on blue jeans) or sewn to a re-inforcing strip of material on the back side of the seam and top stitched then on both sides of the seam.