There are plenty of things it can be used for "in theory". It's making it work in practice that's the tricky part, because if it's done wrong bad things happen.
In a very small nutshell, mRNA basically causes the cells in your own body to produce proteins which they would not otherwise produce. This is where a large part of the problem comes into play, because your body does not like things that are not supposed to be there. Your immune system could recognize cells producing something they're not supposed to be making as infected or diseased cells, and then you die from multiple organ failure as your immune system goes paranoid and trusts no one.
Otherwise, basically figure that it's like telling the body's cells to build things. For instance, say you have someone with muscular dystrophy (Duchenne). It's believed to be caused by a genetic disorder which results in certain proteins not being manufactured properly. With mRNA, you could provide the cells of that person with the correct instructions for that protein. Another such disease might be Haemophilia, where the blood is lacking particular clotting agents, again because of a genetic disorder that causes the body to not create it.
Of course, then we get into an ethical quandry. Sure, we've saved the lives of the persons in question, but we haven't actually cured the underlying cause of the disease -- the genes are still bad. This means that their descendants will likely have the very same issues... which means now the manufacturer of the "medicine" has an almost guaranteed market. Which feels almost more insidious to me than just letting nature take its course.
