Besides lubricants, anti-freeze (as the liquid was once known) contains corrosion-inhibitors unknown to water.
Not sure, but . . . does a 50/50 mix of water and coolant not have a lower freezing point, and a higher boiling point, than water alone?
Oh, Google, wherefore art thou?
At atmospheric pressure, HowStuffWorks tells us:
Fluid - Freezing Point - Boiling Point
•Pure Water: 0 C / 32 F - 100 C / 212 F
•50/50 mix of C2H6O2/Water: -37 C / -35 F - 106 C / 223 F
•70/30 mix of C2H6O2/Water: -55 C / -67 F - 113 C / 235 F
With a pressure radiator, the boiling point rises with pressure.