Straight coolant has a higher viscosity than water and simply doesnt flow well enough on its own to cool. Water also gets rid of heat better. Antifreeze is needed to keep the water from freezing in cold climates. If you live if hot climates, you could run 90% water..
I personally did an experiment last year and went from 50/50 mix to a 25/75% water mix and my temps lowered by about 10 degrees.