Why do we need Quantum theory?

Why do we need to classify drugs in different ways?

Why do we feel cooler under the fan?

Why do we prefer a potentiometer with longer wire?

Why do we apply paint on iron articles?

Why do we prefer mercury in a barometer?

Why do we prefer to use a wrench with a long arm?

Why do not we draw a boundary surface diagam within which the probability of finding the electrons is 100%?