People who operate sewing machines in factories are paid small amounts of money for each seam they sew. More seams=more money. This means that anything that slows down sewing a seam is money lost. For instance, pinning a seam into place, sewing a gathering line (or two!), unpinning, finicking with where a seam should start. All of those are things that mean that the operator is not just sewing a seam together and getting on to the next piece. If they have to do one of those things, someone else has messed up.
This means, that it is likely that you, the home sewer with a few thousand inches of machine operating under your belt, also do not need to have pinned seams and gathering stitches–if you prep your patterns and do your cutting correctly.