Water is essential for life for several key reasons: 1) It acts as a solvent that allows critical compounds to dissolve and replicate, which is necessary for life processes. 2) Water is necessary for metabolic processes like breaking down substances and allowing molecules to form new compounds. 3) Both photosynthesis and respiration, which are essential for producing and using energy in living cells, require water. 4) Water regulates the planet's temperature and circulation of air and nutrients, making life possible on Earth.