So it does not bother to make enzymes that utilize other carbon sources such as lactose unless it needs to. When lactose is present AND glucose is absent. The catabolite activator protein, called CAP, binds cAMP, which then allows CAP to bind the lac promoter and recruit RNA polymerase. However, this will not work if the lac promoter is bound by the lac repressor, which blocks access to RNA polymerase. The only way to get lac repressor off the promoter is to have lactose present.
Combinatorial control allows for the integration of many signals.
In multicellular eukaryotes, gene activation requires the binding of many activators to the promoter region. This crosstalk keeps telling liver cells to be liver cells and not brain cells. As a consequence, cells simultaneously receive and integrate many different signals. To ensure that the proper genes are transcribed at the proper level to generate or maintain the proper cell type, each gene is transcribed only when all the proper signals are present. Each signal might cause an activator to bind to a particular promoter.