Salts,mostly potassium and sodium are known as electrolytes and the body needs them in very specific amounts for proper function. Any significant imbalance of these levels can even be fatal if not corrected quickly.
The normal level of sodium in the body is 135 - 145 mmol/l in the blood while for potassium it's 3.5 to 5.0 mmol /l.
Loss of salts e.g. due to sweating may reduce these levels and so drinking plain water may replenish fluid loss but since plain water has no salts, the salt deficit remains and is in fact aggravated by the increase of plain fluid.
But drinking fluid that has electrolytes both replaces lost fluids and and at the same replenishes lost salts keeping the body functioning well .
