Operons are groups of genes found in prokaryotes, able to be regulated as one unit. The ability to regulate the expression of a group of genes is beneficial, allowing metabolic pathways (among other related genes) to be expressed in unison, when required.
Producing a polycistronic transcript increases the efficiency of prokaryotic transcription, allowing many proteins to be expressed simultaneously. This increases the speed at which protein expression can occur, allowing bacteria to respond more quickly to changing environmental conditions.
For example, the proteins necessary for lactose metabolism are coded in the lac operon. By only expressing these proteins when lactose is present and glucose is absent, the bacteria is able to reduce the metabolic burden of this flexibility. Having the related genes grouped together in an operon also allows for a single regulatory mechanism to control the group of genes, reducing the complexity of the genome – allowing for it to be shortened.