The Marine Corps were formed as a compliment to the naval forces. While the navies fought at sea they still required some link to the land, which became essential for the naval operation success. It was in 1775 that the Continental Congress of the U.S. gave rise to two battalions of marines who served for the naval infantry and it was in 1798 that the U.S. Marine Corp was officially launched. 
In the past the focus of the military campaigns rested mainly on the basis of the naval strength. Land forces came as reinforcement. After the establishment of the Continental Marines the concept of having an elite force that acted completely separately but for the Naval forces arose and this gave birth to the United States Marine Corps. Amphibious in nature the Corp. distinguished itself through many operations and made its necessity felt by the nations. The first raid conducted by the Marine Corps outside its own nations was in the Bahamas in March 1776, under the command of the Corps' first commandant, Capt. Samuel Nicholas. 
Throughout the century the Marine Corps participated in different operations and created a force that was to be feared. The very nature and conception of the Corp. made its importance valid as it was the first group of soldiers that were capable of fighting on land and on the sea.
What was the need for a force that was amphibious in nature? Consider the fact that the wars of the time in which the force was created were mainly manual. They were fought either on foot or through the ships. There were guns and other ammunitions but the fighting took place face-to-face. There were no radars or other such technological devices that would allow the nations at war to keep track of intruders within their lands. The monitoring had to be done by men and the intrusion had to be kept at bay by men. Thus, if the navy fought at sea it needed a force that allowed all the area to be covered so as to keep hostilities away from the mainland. This force came to be the Marine Corps.
When the first group of Marines was created the organizational aspect was lacking. They worked on a ship to ship basis and the actual coordination lacked effectiveness. As the American nation, which had recently got freedom from the British Colonials, fought to remain independent, it realized that the Marine Corps had to be effectively organized at a national level. Piracy was creating a setback to an already weak economy and if no steps were taken there would be fatal consequences. The administrators realized that central command structure had to be built. Congress thus acted immediately and passed "An Act for Establishing a Marine Corps" on July 11, 1798. This marked the first official Marine Corp. And it was seen to be organized into a group of men so that they could fill thirty-two ship's guards. Militarily the hierarchy was based on one major-commandant, thirty-two captains and lieutenants, forty-eight sergeants and corporals, 720 privates, thirty-two fifers, and thirty-two drummers. The Commandant's staff consisted of an adjutant, a paymaster, a quartermaster, a sergeant major, and a drum -- and fife major. 
That two battalions of Marines be raised consisting of one colonel, two lieutenant colonels, two majors and officers as usual in other regiments, that they consist of an equal number of privates with other battalions; that particular care be taken that no person be appointed to office or enlisted into said battalions, but such as are good seamen, or so acquainted with maritime affairs as to be able to serve to advantage by sea." (Resolution of the Continental Congress, 10 November 1775.)
When the naval forces are conducting an operation the marine soldiers operate with the navy on sea and with the land army. They were efficient in their military style and allowed the navy to concentrate on its own tactics while they, the Marines maintained the discipline on board and helped aid the naval land operations. They are considered a separate unit maintained under the Department of the Navy.
The need for a Marine Corp. was simple. The progress seen in military warfare made the society realize that no longer were the territorial waters the main area of war strategies. The campaigns were being progressively conducted within the lands and on the seas. Protecting one area while ignoring the other could cause disaster for the nation and become detrimental in the success of the strategy. It was during the World War II operations that the importance of the Marine Corps became integral and they gained international fame. Responding to the threat from the European and Pacific areas they conducted Dieppe Raid in 1942 and assumed a commando role, which helped the U.S. succeed essentially through the fighting on Guadalcanal, Iwo Jima and the other islands. Marine forces today are considered the creme le de creme of the forces and pride themselves on being the toughest of the tough. 
In hindsight the efficiency of the Marine Corps and their importance was obvious. They were initially not seen as an integral part of the operation but with time a need for their presence was seen and gauged through the campaigns they conducted successfully.
It would seem, particularly with benefit of today's hindsight, that the Marine Corps would be the logical choice for the development of this mission. However, this was not so apparent at the time. 'One school of thought contended that the advanced base function should be performed entirely by Navy personnel under command of naval officers, in the interests of unity and other considerations.' The suggested conflict was based on its theoretical envisage and that is why the Corps was in its inception seen and treated with disdain.
The awareness of Navy's commitments and responsibilities brought about the evolutionary developments which culminated in the early 1940's in the amphibious assault doctrines and techniques "which finally made possible what Major General J.F.C. Fuller has called 'the most far-reaching tactical innovation of [World War II].'"
Through an analysis of the historical military operations and the strategy employed through the focus on sea warfare there was an obvious centralization of control. The military operations were conducted either in the seas or on land. There was remote chance of the two coordinating in their maneuvers. With time this changed and the mission became more critical. The philosophy of war changed, as did its implementation. The heart of the naval capabilities in the operating environment saw a difficulty that could only be solved through innovation. The Marines were that very innovation arising through the ideology of being able to precisely engage the enemy. Land attack encompasses a variety of tasks from long-range precision strikes, to providing precision naval surface fires. The Marines did this. Navy investment in a robust land attack capability from the surface perspective has grown and the precise firepower is exactly what is called for by the Marine Corp.'s operational maneuver from the sea concept. The goal of the Marines is to improve the commander's war fighting capabilities relying first and foremost on the manpower.
As time passed the Marine Corp remained dynamic as it was understood that each new opportunity demanded new thinking and evolving capabilities. Together they it required continued careful planning to balance a whole series of competing priorities: current operating costs vs. long-term investment, for structure vs. combat capabilities, multi-mission vs. core mission capabilities, for structure vs. new technology - these challenges had to be faced. Identifying which capabilities are most critical and balancing that requirement in the near-term vs. those of the long-term is a challenge that the Marines are always ready to face.