If I remember correctly, the Marines use amphibious assault ships that don't have conventional runways (such as on Navy aircraft carriers) to take off/land on thus the need for a VTOL aircraft. A VTOL wouldn't be necessary for a Navy aircraft carrier (example: F18 Super Hornet). Another example of this is why the Marines wanted to keep the Harrier around for so long before it became too problematic and expensive hence why they wanted VTOL capabilities on the F35 JSF (also another factor in the cost/time overruns).