Stem cells are cells wich dont have a function yet. They can become almost every kind of cell (yes cancer too), so if you (as a brilliant scientist), or more simply your body gives a stem cell a function it can repair or edit bodily functions. The injury/disfunction is repaired with fresh cells.
Beside this, new organs and bodily fluids MAY be produced from Stem cells, political and ethical discussion has slowed down the research in this area. Ethical questions are ofcourse needed. Political gain is never a good factor in research.