If your pieces vary in width by 1/8" - 1/16" due to cutting errors, and you sew 20 of them together in strips.
When you go to sew these strips together, your intersections may not line up very well. Each strip you add, will magnify this.
It gets worse when you are piecing smaller block components, say 30+ in a 15" block.
Over all it can case puckering, buckling, and be a pain in the butt trying to get the top put together nicely, and get it to lay flat :D:D:D