Trees need leaves primarily because of photosynthesis and respiration. Photosynthesis is the process by which carbon dioxide is converted into nutrients that are needed by plants for survival.
The leaves of a tree also play a role in cooling down the plant through transpiration. This occurs when water is evaporated from the leaves through the pores. The other role of tree leaves is respiration, which is a very essential process for the plants. During respiration, the pores on the leaves (called stomata) breathe in carbon dioxide and breathe out oxygen; this carbon dioxide is used by the trees during photosynthesis.