The duty of the Marines is to maintain a law and order throughout the world and impose the will and might of the World Government. They are therefore considered the key strategic development force in the World Government and are expected to obey its orders at will. Sometimes these orders are questionable, however they are expected to carry them out regardless of opinion. Amongst their duties is awarding bounty hunters the bounty on the heads of a criminal. They tolerate bounty hunters as a method of making their job easier but are known to go out bounty hunting themselves. Apprehended criminals who are brought in alive are handed over to be made an example of, the Marines are left to ensuring the exportation of these criminals to prison and/or to their execution.
Marines often work in pairs with one superior officer and a second in command or subordinate who is of slightly low rank. These partnerships may remain even during promotions with both pairs advancing in rank together. In the pair the higher ranked officer will give the orders to the men under their command, while the other acts an adviser or information gatherer while ensuring their partners command is undertaken by the men.
Higher ranked Marines (admirals and the fleet admiral) are expected to be on call to lead the Marine forces into war. This may take form of apprehending many criminals at once, protecting the World Nobles or calling a Buster Call to tackle a situation. Vice admirals are the ones who serve on the ships that the admirals call out and are tasked with carrying out the admirals orders exactly.
The main purpose of the Marines remain footed in the idea of bringing justice to any criminal. They use justice as their constituted policy to manage a world that is made entirely of one ocean much easier. However, the main problem within the Marines is the approach to Justice. They are generally divided between those who support moral justice and those who support absolute justice with the latter being the code of conduct for the Marines as a rule. Nevertheless, there is little worry from the higher ranked Marines as to how the Justice is carried out so long as it is, leading those who truly support absolute justice to make unjustified actions in carrying out their duties.
The Marines are also aided by the Shichibukai in terms of maintaining the "Three Great Powers", and as such, are expected to cooperate when necessary
One of the most questionable parts of their "Justice" is when it comes to slavery. Despite the practice supposedly being abolished years before, it is often shown that they just look the other way. Especially when it comes to the World Nobles' slaves, where it is even seen as a crime to free said slaves even if the person is merely a child. Often they are tasked to find and retrieve the person to be a slave once again.