Operons allow expression of a single, polycistronic transcript, controlled by a single regulatory region. Having functionally related genes under the same regulation allows rapid expression of the protein products they code for when required.
The lac operon, probably the best understood operon, is found in the Escherichia coli genome. As lactose is not the preferred sugar for metabolism, the genes for the metabolic pathway do not need to be constitutively expressed. Therefore, the lac operon allows the selective expression of these metabolic genes only when glucose has been exhausted and lactose is the next best option. This allows the bacterium to have metabolic flexibility, while not wasting energy to produce unnecessary proteins.
Operons can have more than one promoter, allowing different sigma factors to express the same proteins. This allows housekeeping genes to be constitutively expressed under different growth conditions, as they are still required for the functioning of the organism.
Operons are important in bacterial adaptation, allowing functionally related genes to be transferred between cells by horizontal gene transfer. This allows other bacterium to take up entire metabolic pathways, which may be useful in certain cell stress situations.