Heating the water for preparation is key for substances to diffuse out of the tea bag (See animation). Diffusion can only take place quickly at a high temperature. The hot water also extracts chemicals within the leaf that give it its flavour and beneficial properties. Boiling reduces the amount of dissolved oxygen in the water, meaning that the leaves will retain their antioxidant properties because they won't react with oxygen. The water is also sterilised, making it safer to drink.