Why should intravenous solutions generally be isotonic?

I have no qualitifcation other than studying science and Biology at high school and university. If a solution is isotonic that means it has the same conecentration of solutes as in the body. This means that when you have the solution in your body salt won't diffuse into your body from the solution or out of your body into solution.

So it seems like isotonic solutions wouldn't mess with your body systems as much which is a good thing.