If the trees are not dumped in the winter the leaves, they would be killed. There are several reasons.
The first reason. The leaves of the tree in the aggregate, have a very large area, and with all this space rapidly evaporates the water. In summer, the tree is able to compensate the loss of moisture by extracting water from the soil. But in the cold cold water extraction from the soil is greatly reduced; in winter from the frozen soil to remove moisture and is difficult. Trees with deciduous cover in winter would die from lack of moisture, is dried.
For the same reason, when the onset of the dry season in the tropics and subtropics trees in these climatic zones shed their leaves and stand naked until the advent of the rainy season.
the second Reason. Have you ever noticed that after a heavy snowfall tree branches strongly inclined to the ground under the weight of the snow? Some branches even break. If the leaves in the winter remained on the trees, the branches were detained would be much more snow as the leaf surface, as we said above, great. Thus, dropping leaves in autumn, the trees protect themselves from mechanical damage under the pressure of the snow.
For the third. During the fall of the tree gets rid of excess mineral salts that accumulate in the leaves throughout the summer. As we have repeatedly noted, the sheet intensively evaporates water. In place of this evaporated water in it constantly receives new, which is sucked up by the roots from the soil. But in the water that the roots get from the soil, various dissolved salts. Thus, the sheets do not get clean water and saline solutions. Part of the salt used by the plant for food, and other salts are deposited in the leaf cells. The more moisture evaporates sheet, the stronger it is mineralized by the autumn. As a result of autumn leaves accumulate a lot of salt, becomes mineralized. Excess mineral salts disrupts the normal functioning of leaves. Therefore, the dropping of older leaves is a necessary condition for maintaining the normal functioning of plants.
By the way...
How do conifers manage not to lose moisture during the winter, staying green? The secret is that the needles evaporated many times less moisture than the leaves. First, the surface of the needles is much smaller than the surface of the sheet; secondly, the needles have a thick skin; thirdly, they are covered with a waxy film, which also reduces the evaporation of water. Finally, the stomata in the needles are in special pits - this reduces the rate of evaporation of moisture through them.
Coniferous trees have shed their cover, but not all at once, as do deciduous trees, and gradually needles of coniferous live, on average, 3-4 years. So change "service" coniferous goes unnoticed.