i think i need to explain my comment
back in the eighties when i done my mechanics coarse the teacher stated that if you was going up to the lakes (our cold area in the mountains) that you needed to change your coolant as the anti freeze component of the coolant would have evaporated out over time as its alcohol based .
now i think David hit the nail on the head when he said its a closed system , back in the eighties cars didnt have closed sealed systems like today , they vented into a vented container or just vented to the atmosphere where now they are sealed so no evaporation can happen. while this may have been relevant then its probably not so much today . back in the days of just using water from the tap and the use of coolant started to be used because things were being made from aluminium, water pumps, heads etc and the water just ate out the aluminium parts. its only in extreme cases that you need to use anti freeze anti boil products instead or just an inhibitor (i think some of yous live in these extreme areas)
PS i only use inhibitor and have never had over heating or freezing issues ever, also never had issues with corrosion of any description.
i hope that makes sense of my comment.