Plants are modular in construction.  They grow and adapt by making new modules.

Often the design is gradually or suddenly varied to deal with the challenges of a changing environment  (for example development of dormant buds for winter survival) or to meet life-cycle needs (for example flowers for reproduction).

Growth, adaptation and life-cycle events are usually accompanied by a more or less casual abandonment of existing modules (as happens during autumnal leaf-fall, for instance).

Although this appears to be dangerously wasteful of precious resources, it turns out to be an effective way for sedentary organisms to behave, particularly as the capacity to photosynthesize relieves plants of many of the energy constraints that animals have to contend with.

Discarding leaves and other modules is only sustainable if there is an efficient way of recovering and reallocating nutrients, such as nitrogen or phosphorus, that could be in limited supply.

This is where the recycling function of senescence comes in.  You can read more about this in H Thomas, V O Sadras (2001) The capture and gratuitous disposal of resources by plants. Functional Ecology 15: 3-12