My DI (Drill Instructor) in 1950 said it was to "Seek out the enemy and destroy him."
I accepted that premise, I also learned that the "Buddy on your right, and the Buddy on your left are your responsibility" from people like Chesty Puller and Victor H. (The Brute} Krulak.) Accepted! Understood! (And thus you survived for one more day to seek out the enemy and destroy him.) (If that sounds like "Beer Talk", it probably is.)
As any Marine knows the "Mission" of the USMC is to "seize and defend advance navel positions" quoted right out of the doctrine.