Antifreeze/coolants are vitally important to engine health. It undergoes constant temperature changes and is vulnerable to cold, heat, and the passage of time. It needs antifreeze/coolant fluid to maintain an adequate temperature, helping to prevent the engine from being damaged by temperature variation, both outside and inside the vehicle.
Although water is a priori the best coolant that exists, given its extraordinary heat capacity, we must bear in mind that, in addition to corroding metals, water boils at 100º and freezes at 0º, and the temperatures that a vehicle can withstand are in some cases higher than 100º and in others lower than 0º.
Taking these factors into account, a chemical base is used (generally based on monoethylene glycol) diluted in water and with anti-corrosion and anti-cavitation additives and inhibitors. This lowers the freezing point, increases the boiling point and protects against corrosion and oxidation. They also reduce the formation of deposits protecting all the components of the cooling circuit.
Depending on the proportion used in the solution (glycol+water+additives+inhibitors), we can speak of antifreeze/coolants with concentrations of 20%, 30%, 50%, etc ... The higher the concentration percentage, the higher the boiling point and the lower the freezing point.
In general, a bad antifreeze/coolant with an inadequate or poorly formulated corrosion inhibitor package will cause corrosion on important parts and thus mechanical failures. These will start in the cooling system (thermostats, radiator, water pumps, cylinder heads, heating circuits, etc.) and then in the car's engine system.