When it comes to the shirt fit is one of the hardest parts to get to the back. Many people have experienced that the shirt fits well over the shoulders and shoulder blades, but feels like a tent over the middle and lower back. The best solution to overcome the problem is to choose a shirt with darts.
Why not just sew up the shirt at the waist for a slimmer fit? It might work if the man was completely flat, but a good fit requires more contact points to accurately able to follow the body without compromising the garment’s mobility. Looking at the body in profile, you realize quite easily the difficulty in compensating the spine concave shape by only sew the waist. Vertical incisions however, takes away excess fabric from the middle and lower back while mobility has improved by two additional contact. The principle is actually very similar to the jacket vents, except that the jacket has open slots.
Another advantage of the incision is that this is a relatively simple measure to adjust a slightly too generous cut shirt of change tailor. Just remember not to overdo the incision, which can give a somewhat feminine shape that clearly marks the hips.