they site inject to fix symmetry discrepencies. Many people have slight differences form the left to right sides of the body, as well as differeing nerve assemblies. When you use hormones to grow, everything grow, and the errors become more obvious. shots can reduce that.
Look at Phil heath. He's blowing up, and should be top 3 at the O in no time. But his perceived weaknesses - the narrow shoulders and bunch-y chest - those are going to become more obvious as he grows. Not taking anything away form the guy - he'll be top 3 in the world for a long time with those arms. But his imbalances are something that will haunt him, as they do with most bodybuilders.