Some plants have thicker leaves, like magnolia. This will allow them to intercept as much light as possible and to prevent other plants growing under them. Leaves are also important in our environment. While on the plant, they provide cooling, shade, and water, air, and noise pollution control. This system allows air to get into the leaf to provide the carbon dioxide necessary for photosynthesis.
Water is also important.
This is called transpiration. Water is also used in photosynthesis. If there is not enough water, leaves close their stomates and wilt to control water loss. Wilting is a sign that the plant is in trouble. Waxes and hairs on the leaf keep it from drying out.