Photosynthesis in plants from C02 and H20 in the presence of sunlight and green colouring material called chlorophyll to form carbohydrates and supply of dioxygen is an important usefulness of water.
Similarly glucose metabolism in our body to give out C${{O}{2}} and {{H}{2}}$O vapours with the release of energy is an important application of water.
The above reaction is exothermic. Energy released is utilized by human body to do work.