Protective layer of oxides on the surface of metals help in:

In normal temperatures or normal conditions, the surfaces of some metals remain covered by a thin layer of metal oxides.Due to the presence of thee metal oxides, metals lose their lustre. Also, This layer forms to protect the metals from further oxidation.

For example, we need to remove the layer of magnesium oxide from the magnesium ribbon by scrapping as magnesium oxide forms a protective layer and decreases the reactivity of magnesium.