What is the evolutionary purpose of allosteric binding sites on receptors. did they have endogenous ligands at one point?

Good question, No.: Study: http://en.Wikipedia.Org/wiki/allosteric_regulation since protein function is a result of structure & subtle yet precise changes in moleculrar shape, allosteric binding sites simply refers to the many locations on proteins which can be bound/altered in some way so as to shift molecular shape changes, thus function(s), regardles their genetic/history/evolutionary reasons for existing.