By sending an astronaut with diabetes into space, the life sciences experiments get accesss to a very special laboratory. Type 1 diabetes creates a body where certain automatic functions are missing, and thus the researchers get “manual control” over blood glucose levels. This can affect various bodily functions and so effects of medicine and/or activities can be tested.
Furthermore, testing medicine, primarily insulin, but also experimental treatments, in microgravity can provide additional information, that otherwise could not be obtained on Earth.