What is the purpose of operons in prokaryotics?

An operon is a means of a controlling the expression of genes in prokaryotes. Prokaryotes often cluster genes that code related proteins in the same area. When these proteins are required, prokaryotes activate the operon and these genes are transcribed. However, when these proteins are not needed, a repressor attaches to a site on the operon, and none of the genes are encoded. 

The most common example of an operon is the lac (lactose) operon, which controls the degradation of the milk sugar lactose. When lactose is present, it binds and causes a repressor to detach, so the proteins that will metabolize lactose can be transcribed. 

There are three proteins involved in the lac operon, lac Z (which encodes the protein beta-galactosidase, that cleaves lactose into glucose and galactose), lac Y (encodes permease, which pumps lactose into the cell) and lac A (encodes transacetylase, which transfers an acetyl group from acetyl-CoA to beta-galactosidase). When lactose is present, the operon becomes activated, and all three of these genes are transcribed, producing the necessary proteins needed to metabolize lactose.