Plants do not carry pathogens that might be dangerous to human health. Additionally, on the level of pharmacologically active proteins, there are no proteins in plants that are similar to human proteins. On the other hand, plants are still sufficiently closely related to animals and humans that they are able to correctly process and configure both animal and human proteins. Their seeds and fruits also provide sterile packaging containers for the valuable therapeutics and guarantee a certain storage life.
Global demand for pharmaceuticals is at unprecedented levels.  Expanding the existing microbial systems, although feasible for some therapeutic products, is not a satisfactory option on several grounds.  Many proteins of interest are too complex to be made by microbial systems or by protein synthesis. These proteins are currently being produced in animal cell cultures, but the resulting product is often prohibitively expensive for many patients.  For these reasons, science has been exploring other options for producing proteins of therapeutic value.
These pharmaceutical crops could become extremely beneficial in developing countries.  The World Health Organization estimates that nearly 3 million people die each year from vaccine preventable disease, mostly in Africa.  Diseases such as measles and hepatitis lead to deaths in countries where the people cannot afford the high costs of vaccines, but pharm crops could help solve this problem.