Why are there multiple oxidation states?
Because the same elements can react with each other in DIFFERENT proportions...
The oxidation state of an element in a chemical compound or as the element is of course a formalism, however, it is a very useful formalism that allows us to balance quite complicated redox equations straightforwardly.
The oxides of nitrogen provide a nice array of oxidation states.
And of course in this series the oxidation number of oxygen is constant at