Many vehicle users do not see the need to use coolant solution in their vehicles simply because they put in water and their vehicle runs fine.

This is because they are unaware of the long term negative effect that it will have on their vehicle's cooling system and the engine.

HOW COOLANT PERFORMS WHEN USED AND HOW WATER PERFORMS WHEN USED IN THE COOLING SYSTEM.

Plain water used as coolant

This could be either tap, well or borehole water. These types of water contain minerals that cause corrosion to a car's engine block, piping and the core of a radiator. Those minerals can also cause scale formation in the cooling system.
An example of where one can notice scale formation is in the home electric kettle, you will notice a brown scale inside the kettle after some use. That is scale! When water that contains these salts is heated and then evaporates, it leaves behind the salts that form scale. It is this same phenomenon that occurs in the cooling system of a car.
Though the use of water is not encouraged, the impurities help to raise the boiling point of water.

Water is not good to be used as coolant in the engine because of the damage caused by those minerals as the minerals will eventually block passages, affect heat transfer from the engine block to the coolant. Scales and those mineral buildup can block passages in the radiator specifically since the radiator has narrow passages. It can also make the Thermostat to become stuck closed or stuck open or stuck partially closed. This can have minor to severe effect on the engine as a result of overheating.

Demineralized water/distilled water used as coolant

This does not contain those minerals that cause corrosion and scale. A cheap source is water that comes out of air conditioning system.
One advantage of using demineralized water over ordinary water is; there won't be scale formation with its use and the water will boil at normal boiling temperature of pure water. Using distilled or demineralized water is not also very good as it is bad for overheat protection because once the temperature reaches about 100 degrees celcius, it begins to boil especially when the cooling system has lost pressure due to faulty radiator cap or leakage somewhere. Some vehicle's normal operating coolant temperature are as high as 105 degrees celcius.

Coolant solution used as coolant

The major content of coolant solution is mostly ethylene glycol. Its boiling point is high than that of water and its freezing point is low. This helps to give the engine high temperature protection, easy start and protection in freezing temperatures. Coolant solution also contains corrosion inhibitors that prevents corrosion and it is also used as lubricant for the thermostat and water pump.
As you can see, coolant solution is the best choice among ordinary water and demineralized water.