Water is ubiquitous, and plays more roles than I can count. It cycles heat from the surface into the stratosphere and then back, allowing the Earth to cool off. It changes the albedo (the amount of light that gets reflected back into space) everywhere. It is the Earth's most important greenhouse gas. It is the main solvent for all life.

Water is necessary for life on Earth. We don't know if there could be any type of life on a planet without water, but it would be a very different type of life from life on Earth.

Here is a link: "These properties include water's exceptional capacity to absorb, store, and release large amounts of energy, transmit sunlight, expand upon freezing, dissolve and transport materials, and lower the viscosities and melting points of rocks."