As water is hypotonic to cells, the blood cells, that is, in the absence of cell wall will continue to enlarge until they get burst. In case of a dehydrated patient, one hydrates them with the help of drip feed. If one is giving them hypertonic saline, it would be like consuming seawater, and the cells would dehydrate more as they would get into the bloodstream.
In case, if one is administering them hypotonic saline, then the cells of the blood would absorb more water and get burst. Thus, there is a need to present the patient with isotonic saline drips.