If body fluids are lacking due to losses and/or increased demand, they should be restored with (balanced) electrolyte solutions.
Electrolyte solutions are given to cover basal fluid demands or compensate for fluid losses. In addition, balanced electrolyte solutions provide diverse electrolytes targeting to maintain/restore a physiologic acid-base-status.
Furthermore, electrolyte solutions are used as a carrier for several drugs (e.g. i.v. antibiotics). Glucose solutions are part of a sufficient parenteral nutrition therapy.
Remember: 50-60% of our total body weight is water and electrolytes.