Why use neurotransmitters?
Your brain uses over 50 different neurotransmitter chemicals. Although electrical signalling between neurones is quicker and more energy efficient, chemical signalling is far more versatile. The signals carried by some neurotransmitters excite the target cell while others dampen down their activity, depending on the type of neurotransmitter released at the synapse and the receptors they reach. This is what sharpens the contrast between light and dark in the eye, for example.