Why can water be used to put out fire

Often, the main way to extinguish a fire is to spray with water. The water has two roles:
1. In contact with the fire, it vaporizes, and this vapour displaces the oxygen (the volume of water vapour is 1,700 times greater than liquid water); leaving the fire with not enough combustive agent to continue, and it dies out.
2. The vaporization of water absorbs the heat; it cools the smoke, air, walls, objects in the room, etc., that could act as further fuel, and thus prevents one of the means that fires grow, which is by "jumping" to nearby heat/fuel sources to start new fires, which then combine.
The extinction is thus a combination of asphyxia and cooling. The flame itself is suppressed by asphyxia, but the cooling is the most important element to master a fire in a closed area.
asphyxia - condition of severely deficient supply of oxygen

I hope this can help your understanding. =)

Because water seems to cost nothing and very abundent on earth.