It makes for better unit cohesion - Marines can train with Marines, and at least in theory everyone will eg know everyone's radio freqs and call signs and so on

It makes for better unit cohesion - Marines can train with Marines, and at least in theory everyone will eg know everyone's radio freqs and call signs and so on. (In practice, lol no not even close) Also specific to the Marines is the idea of delivering a fully-operational fighting force anywhere in the world in a short time: infantry, armor, artillery, CAS. Marines even have their own specops (Recon). So while the Marines rely on the Navy for transport (the Marines are a part of the Navy, after all) their land warfare doctrine calls for combined arms. Can't have CAS if the wingwipers are busy doing top-cover 100 miles out from your beach assault.