Why are resonance structures used instead of lewis structures to correctly model certain molecules?
Structures and diagrams are representations; they are not the molecules.
However we represent the molecule, we are not going to change its formula, and we certainly are not going to change its chemistry. Nevertheless, as chemists, as physical scientists we want to make sure that we are not misunderstood, and able to communicate with our colleagues unambiguously.
The benzene molecule is probably the best example of this. Most older generation chemists would be quite happy to represent this as a cyclohexatriene with