Alcohol also serves to prevent freezing of the fluid which is a concern for most people that live more than about 35 deg from the equator.
Alcohol also serves to prevent "crud" from growing in the water mix due to any contamination of the water by alge or fungi.
It also makes leaks more apparent as you can easily smell a leak in a water/alcohol system but a leak in a water only system may go unnoticed until rusting, mildew or other problems show up.
Alcohol also reduces the changes in ignition timing that occur with straight water, and in some setups serves as additional fuel if the stock fuel system if limited in its maximum flow capability.
Alcohol also greatly improves evaporative cooling in high humidity environments where the atmosphere is normally very humid.
Each car is different and most users tinker with the mix to find the best for their needs.
The military generally settled for a 50/50 mix of water and alcohol in the military aircraft of WWII as it had the best combination of characteristics for their needs. That mix has become more or less the standard starting point for many as its effects are very well documented.
Larry
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