More or less the definition of a state is that it upholds the monopoly of violence within its borders. In modern liberal states that monopoly comes with extensive regulations on how, when and by whom violence should be used. This is called the rule of law. The regulations are usually broadly enumerated in the constitution or similar basic law, and in more detail in ordinary legislation. In democratic states these are ideally determined by the population, usually via some representatives.

Sorry if it sounds like I'm stating the obvious, but this is the rationale why many people think that some powers may be granted to the institution with the sole legitimate power to force you to comply at gunpoint.