As plants grow new leaves the older ones are dropped to create more space. It is essential to do this as the efficiency with which older leaves can make food is reduced due to damage caused as they age. New leaves can produce more food using lesser resources.
Some types of plants, like deciduous plants, pines and firs drop a large portion, in many cases all their leaves during months when the amount of sunlight available is very less and temperatures drop to very low levels. During the cold months, there is no production of glucose in the leaves as chlorophyll requires sunlight as a source of energy. The plants survive on food that has been produced over the rest of the year and is stored in their trunk and other branches.
Leaves are also dropped to reduce the surface area, so that the plants can bear more extreme temperatures before they freeze. In several plants a large amount of the nutrients present in the leaves is extracted before they are dropped. Even in plants where this is not done, the nutrients in the leaves are available for them when the winter ends and they start to grow a new foliage.