Why we should add Distilled water to a CAR battery?

It helps increase battery life because the liquid in the battery dries up over time. If you want to preserve you car battery add distilled water once a year. This maximizes battery life. However when you put distilled put petroleum jelly on the battery connectors (the + and -). This prevents them from rusting and corroding.

When battery water is low, the anode and catode battery plates in insuffient submerged. We have to top it up. The best is to add pure (de-ionized) water. It is less available compared to distilled water. Adding ions (minerals) into battery would combine with battery water (sulfuric acid) to become a sulfate salt. Tap water have metal from rusting pipe. Non-RO bottle water would have some dissolved minerals. *My mechanic however advice that the best is to replace the battery water from another battery. Usually there is a high low marker at the side of battery. Do visit the links below.

People are dancing all around the correct answer. Simply put; because anything but distilled has minerals in it that will accumulate on the battery plates reducing their effectiveness. It has nothing to do with terminals and you don't ever want to drink battery water.

Because distilled water has no contaminates, minerals etc in it that could deteriorate the lead cells. Minerals like calcium can coat the cells in the battery and after time slow the flow of electric current.

Regular water has minerals in it. It will coat the internal terminals of the battery and it will not be able to produce an electronic current. It will be shorted out. The battery will be dead.

distiled water is pure H2O without mineral specification

Mineral water has, well, minerals, which have ions that could cause reactions with the H2SO4 hydrogen sulfate in the battery acid or upset the pH balance.

i think it has something to do with the water being free of impurities

Just in case you break down in the middle of the desert you've got a spare supply of drinking water

because it's pure water i thought that might be pretty obvious