/u/Schaftenheimen has previously answered Why was the USMC such a small military branch before 1941? which explains some of it.
/u/pnzsaurkrautwerfer has offered a relevant answer to Why does the USA still have Marines?, among others.
/u/Tacklos discusses Marine Corps mythos in Who was the first person to be called "Commandant of the Marine Corps"....?
I collected other answers about "Why didn't the Marines fight in Operation Overlord/ETO?" in this thread.