Water is used in car radiators as coolant because of

$Q=mc∆T\to ∆T=\frac{Q}{mc}$

The more the specific heat capacity of a substance, the lesser is the rise in temperature for a given amount of heat supplied to a substance. We want water to absorb heat from hotter parts of the radiator but at the same time, water itself should not become very hot. Otherwise it that may be damaging/dangerous to the car/people. Since, water has high specific heat capacity, its temperature rises moderately.