Tone matters in the body

Your blood and tissue fluid carry dissolved salt at a steady strength. Plain water on its own is weaker than that, and a red cell sitting in plain water swells and bursts. The label states this plainly: water given into a vein without a solute can break red cells. Once a powder is dissolved in it the mixture carries that solute, and the small volume under the skin is diluted by your own tissue fluid within moments.