Why are resonance structures necessary?

Resonance structures are a way of depicting the concept of delocalised electrons (electrons that are not uniquely associated with a specific atom or bond) in the context of valence bond theory using standard skeletal or line-angle structural formulae.

Delocalisation of electrons over more than one atom or bond explains additional stability that would not be observed if the electrons were localised on a specifc bond or atom. For example, if we take the nitrite ion, the electrons are delocalised over the O-N-O structure, Valence bond theory, doesn't account for this, and would require the structure to be represented as either:

a) #O=N-O^-# or b) #O^_-N=O#

In reality the structure is neither of these, but something sort of "in between". Resonance structures allow us to depict it, so we would write the above two structures with a double ended arrow between them. The arrow denotes that the real structure is a "resonance hybrid" of the two above structures.

It is common for students to misunderstand this concept, due to the term "resonance", and to assume that the structure somehow "flips" between the two structures shown. It doesn't, and the word "resonance" is just an unfortunately confusing coincidence!: