Why are Marines deployed in the desert?

The US military operates on a "joint forces" concept. That means we are more accurately referred to as US Armed Forces. Nobody operates alone. Each branch supports the other. The marines are a combat operations force, and as such, were utilized in that capacity. They, as opposed to the Army, have a more specialized mission because they have their own air force. The Army has a fleet of choppers, but don't employ fixed wing aircraft. Air support primarily falls on the Air Force, as far as fixed wing assets are concerned. The Marine Corps, I'm sure, prefers to use their own air assets. So, in the desert, the Marines were utilized to augment Army personnel to prevent the Army from being spread too thin. It didn't make much sense to have this highly capable and extremely specialized combat force just sitting around on their hands while there was a big land war going on.