Why does intravenous fluids have a solute concentration isotonic to that of blood's plasma?

Isotonic means that the solution is equal to the tonicity (and/or osmolarity) of the part of the body you are administering it in. In simple terms, this means that the solutes of the drug/solution equals the solutes of the blood, eye, ear, etc. For example, the eye has a different tonicity and osmolarity than the blood, thus you must account for this when administering a medication.

It is important to make sure the solution is isotonic for the comfort of the patient and to prevent tissue damage. If the tonicity doesn't match, it can be very painful because the cells may be exposed to too much or too little solute, thus water will be exchanged very rapidly and could cause cell damage.