Part of the capabilities of the Marines is being able to move quickly and rapidly deploy anywhere which is useful in a shock or front line component to move in in front of slower sustainment forces like the Army.

They are assault/shock troops.

The Army has a sprawling operation and mission, but the Marines focus around that shock infantry model. The entire organization is focused around that, which has good and bad attributes.

Let's put it like this: The South China Sea isn't called Sea because it's landlocked.

“Generation kill”, the book and the miniseries, is about marines invading Iraq in 2003 in unarmored humvees. Didn’t make sense then either.

The USMC has long been fighting for relevance. Fighting in Afghanistan was simply them doing literally anything just to do something

The Marine Corps proposed to take over most of the fighting in Afghanistan: https://www.nytimes.com/2007/10/11/washington/11military.htm...

Agree that it was playing the game to justify its existence. Personally I think "the nation's ready response force" is a good and necessary mission.