What is the major advantage of an operon for a bacterial cell?

All of the genes needed for a particular function or a group of similar functions are all next to each other.
Most studied is the lac operon;. This is for the digestion of lactose.
Bacteria produce there mRNA's in apolycistronic form, which is, put all of the operon genes on one RNA and translate all of them at the same time. Unless some part of the gene is turned off.

good luck.

The main advantage is that the various proteins that are build from one DNA operon emerge spatial and temporal coordinated and often in the right ratio needed for building up a multi-protein structure.
Just assume you have a 4 different proteins A,B,C and D that are forming a complex protein structure. If all of them would be located on different mRNA strands, the would might be translated into protein at different places at different times in the cell. They would need to find their matches for building the protein complex. Thus organizing all of them in one operon i.e. on one mRNA is a major advantage.