Whats the purpose of allosteric binding sites on receptors... do they have an evolutionary role?

Allosteric sites: Receptors can be regulated through allosteric binding. The production of enzymes and other proteins can be affected in this way, and don't have to rely only on the main active site. Some substances might be allosteric activators, and others allosteric inhibitors. Nature usually provides multiple pathways for vital metabolic processes to continue, in a fail-safe kind of way.