It is accompanied by 'producer' cells and 'helper' viruses that enable it to grow and multiply, even though the virus that will carry the gene is incapable of replicating itself in live organisms. This means you can get a lot of identical viruses, each one of which will infect one cell. If we can get enough copies of the virus, in theory, we would have sufficiently high numbers to reach a high proportion of muscle cells, even though the infection can't spread from one cell to another. This all seems a very complex way of getting the dystrophin protein into muscle. You don't just need to get this protein in, you have to insert it in exactly the right place within the cell structure.
Cells have their own processes to deal with proteins they manufacture for themselves.
If dystrophin was simply injected into muscle it wouldn't lock into the cell's own protein processing system. Proteins that aren't properly processed are left open to being damaged by the chemical environment and will be destroyed. Scientists have thought for a long time that viruses are a good way of moving genetic material into cells. By the time our researchers were ready to try and get artificial dystrophin genes into muscle, viruses had already been developed, but they weren't specifically designed for muscular dystrophy. Using these 'off-the shelf' viruses, high dystrophin levels were achieved, initially in cell culture and animal models.