The Marines dating back to the Royal Navy were originally to serve on ships. They protected the officers and were used to great effect in ship to ship action when boarding. Between world wars they had justify their existence and became specialists in over the beach invasions. That has become their main claim to fame. Of course you can ignore the huge invasions over the beach conducted by the Army in WWII. Think SW Pacific, Philippines, Okinawa, North Africa, Normandy, Sicily, Italy, and southern France.