My understanding of the need/benefit for IA is that injecting into the synovial fluid achieves a far higher local peak concentration of GH (and hence saturation of local receptors) than could be achieved if systemic GH had to seep into the synovial fluid. That said, acromegaly and joint crowding from chronic high-dose systemic GH is certainly a thing ( and what started Dr. Dunn down this path) but it comes with insulin resistance, unwanted joint growth in healthy joints, carpal tunnel, a much longer timeline, and all the other problems that come from excessive GH.
The back is a different story since AFAIK, with the exception of the disks themselves, there really is no equivalent of the joint capsule to inject into, so systemic is the default if one is going to use GH. Also, with joints you want a degree of tissue growth (cartilage) however from what little I know about the back, I can see generalized growth (as opposed to localized healing) being a potential problem since it might created pressure on nerves.
The biggest factor that I can identify in the improvement in my back is reducing the situations where I load my spine in a non-neutral position. That means minimizing things like weighted side bends, lowering my bike seat, reducing twisting motions, etc.