Many often accuse ethics of being devoid of substance; a purely subjective study. The fact is that they have failed to examine what ethics genuinely is.
Ethics, properly considered, is the study of conflict in human relationships. Think about it; if you aren't involved in conflict with anybody, then there isn't any legitimate objection to your behavior. Ethical action is interpersonal behavior which does not generate conflict, and unethical action is that which does. Conflict is a physical event which is objectively observable, and responsibility for conflict is objective and inter-subjectively verifiable. Thus, a properly-formulated study of ethics concerns real-world events and has very real implications.
The Non-Aggression Principle is extrapolated from ethical concepts and is a useful heuristic for identifying ethical and non-ethical action, and for determining responsibility where conflict has occurred.
>Why should I care about NAP?
Because if you don't feel like studying ethics directly, it's the most useful tool you have for pursuing conflict-free, and thus more efficient, interpersonal relationships.