Many people assume that coolant/antifreeze is only used to prevent freezing in cold weather.
In addition, the special additives in the coolant have to prevent the engine’s internals from corroding and also supply lubrication to moving parts, for example, water pumps and HCV's. Just as cholesterol blocks human arteries, corrosion clogs the cooling/heating passages and components, reducing their efficiency.
These special coolant additives diminish over time and can lead to problems.
It's better to change the coolant fairly regulary to maintain this protection, as just checking the coolant concentration does not show any additive deterioration.