Hanging upside down has saved many people from back surgery, but it is one of the best preventative practices you can do for spinal health.
Although this is purely anecdotal, I have noticed that the back exercises I had been doing, which consisted taking sidesteps with a big rubber band around my knees, became much easier. Before changing I could do 50 left and 50 right steps before my back muscles were fatigued and cramping to the point where I could barely stand. After 2 months of hanging (and NOT doing ANY back exercises), I found I could to 50 left and right with barely any discomfort! I am sure I could do 150 now, but I haven't tested that yet.
I did a bit of research into this and found that yes, stretching alone can build muscles (or at least help them build, which means if you stretch, you still have to use the muscles in some capacity. For me, that exercise is tango dancing, which is surprisingly demanding on the back muscles). More here at http://stretchcoach.com/articles/stretching-for-muscle-growth/
In my case, I saw at least 100%, if not 150%, improvement in muscle ability, just from hanging!