Many people use pure water as radiator coolant. This results in rusting which works as an insulator in the radiator so that the heat is not taken out as is required. This means the engine temperature operates above the designed temperatures thus shortening the life of the engine. The rust can be deposited on the thermostat and therefore the thermostat does not regulate the engine temperature properly. The rust could also be deposited in the cylinder head causing the uneven expansion/contraction of the cylinder head and untimely cracking which results in water flowing in the engine causing bearing knock.
Engine coolants are a blend of soft water and Neat engine coolant that makes the boiling point of the coolant to be higher than that of water. This enables the coolant not to boil even when the engine is overworked. It protects radiator from rusting thus making the cooling efficient. It keeps the thermostat clean so that it can effectively control engine temperature.