Why is the layer of Zinc coated over Iron?

Galvanization is a metal coating process in which an iron part is coated with a thin layer of zinc. The zinc coating seals the surface from the environment, preventing oxidation and weathering. 

It extends the life of iron parts by forming a membrane between the metal and the air, preventing the accumulation of iron oxide as Zinc is more reactive than iron, so it oxidises in preference to the iron object.