Why have both an endocrine and a nervous system? Although we have not yet studied the endocrine system, you know enough about it to answer the question. Both systems use chemical messages (hormones or neurotransmitters) and they are often identical molecules. The endocrine system releases hormones into the bloodstream and they are carried everywhere. The nervous system stores neurotransmitters at nerve endings in synaptic vesicles and they are released at a very specific location. (You may want to read some basic information about each system.) Speed of action is an obvious answer, but it is not an acceptable answer. Neither is duration of action (both can cause long and short term effects). There is a more important difference in the action of the two systems. To answer the question, consider how each system would do some simple job normally done by the other system (it can be done). You can assume that any neurotransmitter could be a hormone (stored and released by an endocrine gland), and that any hormone could be a neurotransmitter (stored and released from synaptic vesicles).
One simple perspective on this is the distance the message travels within each system and how specific the message is meant to be. Neurotransmitters travel locally (neuron to neuron, or are released into a discrete brain area) ...
This solution justifies why we have both an endocrine and a nervous system.