What is the function of a cascade of phosphorylation in cellular signal transduction?

Signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another

Most processes of signal transduction involve ordered sequences of biochemical reactions inside the cell, which are carried out by enzymes and activated by second messengers, resulting in a signal transduction pathway. Such processes are usually rapid, lasting on the order of milliseconds in the case of ion flux, or minutes for the activation of protein- and lipid-mediated kinase cascades, but some can take hours, and even days (as is the case with gene expression), to complete.

[Animation] - Signal Transduction by Extracellular Receptors