i) Old and inefficient organs are replaced by young and developing organs.
ii) Leaf fall in deciduous trees reduces the rate of transpirational loss. This is an adaptation to tide over winter.
iii) Recovery of nutrients from senescing organs by diverting the nutrients to the younger parts where they are utilised as reserve materials.
iv) Leaf fall adds leaf material and its break down products to the soil, which are again used by the plants for their growth.
The process of separation of leaves, flowers and fruits from a plant is called abscission. When these parts are removed, the plant seals off its vascular system to prevent loss of water and nutrients and excludes bacteria, fungal spores and other pathogens. An abscission zone, a layer of specialised cells forms at the base of each part before it is lost, to separate it from its main body. The cells of this layer die, and become hardened by the deposition of lignin and suberin. So by the time the leaf or fruit falls off, the vascular system has been completely sealed off.
Abscission is controlled by the hormone ABA.