Cheetah
Deity
Several replies, but not any serious ones.
Marines exist because their methods and doctrine contain options and abilities that the Army doesnt have.
Marines were meant to be shock troops to assault beachheads, survive and achieve the goal. As a whole, they are generally viewed as a higher trained force in comparison with your regular army infantry unit, on par (more or less) with the Army Rangers (another specialized unit).
The tradition of having marines is most prevalent in nations with large maritime histories. Again, it takes a different skill set to defend a ship effectively, and/or be able to assault a defended beach from the sea.
Now that is a very, very small synopsis on something you could probably write a whole dissertation to. So what you get out of that may vary.
So can I boil all this down to: the Marines are a separate branch because of history and tradition, and paratroopers and other specialised units came to late to the party to be their own branch?
Cause it really sounds to me like the Marines should just a an amphibious special unit within the Army, just like there are Rangers, Green Berets, etc.
Also, could you (or anyone else for that matter), give some comments to my follow-up question? Do you think the way the military is organised now is the best way, or can you see other ways that would be more optimal (feel free to define the military's objectives as you see fit)?