We would never say that glutamate or GABA, the “basic” neurotransmitters, have a particular function. So why do attempt to give modulators like dopamine or oxytocin a defined role? Dopamine, for instance, is not only spread across the brain, but is also in the retina!
Do either dopamine or oxytocin – or any other neuromodulator for that matter – have a unifying function? Or did the brain evolve multiple independent uses for the modulators?