In addition to lowering the freezing point, making the eventual ice slushier, and otherwise modifying the low-temperature expansion chacacteristics, antifreeze does two other things for cooling systems:
- It raises the boiling point, allowing the engine to run hotter without boiling over.
- It has additives that inhibit corrosion of the metals in the water jacket. (This lengthens engine life, improves heat transfer, reduces blockage of cooling water passages and jamming of thermostats by rust flakes, and the like.)
You should use antifreeze even in the summer and even if you're not pressurized. You should also change it every couple years to renew the corrosion inhibitors (and flush out any corrosion and debris that occurred regardless).
They now call it "antifreeze and summer coolant", in an attempt to keep car owners in mild climates from refilling with water (especially in the summer), skipping cooling system service, and the like - eventually destroying their engines as a result.
