Why does the U.S.A need 4 Branches of the Armed Services? (ARMY, Air Force, Navy, Marines)

Because we can only count to four? How about the Coast Guard is #5? We tend to forget them, and Merchant Marines, etc. I'd go for one common purple suit if we could agree on one like the Confederation in Star Fleet....many specialties...if we could manage it. One set of ranks, etc.

There's actually 7 uniformed branches. Also, the CG is an armed branch.

4 different mission sets to address evolving problems on and off the battlefield. You mentioned redundancy, which can seem like a crutch more than anything when considering the other branches, but a certain level of redundancy to me seems to allow each branch to at least address those problems without reliance on another. Also, autonomy comes to mind. Not autonomy in the general sense of decentralized command, more so the autonomy to allow each organization to function using its own set of prerogatives on the same battlefield. I'm just an E-4 this is beyond my pay grade, haha.

The Canadians tried this for about (or aboot) twenty years, then went back to the traditional RCN, RCAF, etc. IIRC, troops' morale improved, as did their effectiveness. For us, South of the Border types, there is much more to efficiency than saving a few dollars on uniforms, training and equipment. I'm old enough to remember McNamara's band, and all of the difficulties that gang caused, so when this topic surfaces, every generation of so, I just want to hold my head and rock slowly back and forth, back and forth.....

Brig. Gen. Victor Krulak, in 1957 was asked, “Why does the U.S. need a Marine Corps?” Krulak, already a legend in the Marines, penned this reply: “In terms of cold mechanical logic the United States does not need a Marine Corps because she has fine modern Army and a vigorous Air Force and Navy. However, the Americans want a Marine Corps for three reasons…they believe when trouble comes to our country there will be Marines to do something about it….Marines always turn in a performance that is dramatically successful….they believe that the Marine Corps is downright good for the manhood of our country. We [the Marine Corps] exist today—we flourish today—not because of what we know we are, or what we know we can do, but because of what the grassroots of our country believes we are and believes we can do.” This statement is as true now, as it was then.

Form, function and thought process. The different focus that each service tends to mold the service members differently. It is easier to pick from a collection of mindsets that suit your intended outcome. The Marines and Air Force Branched off from there parent services because they needed a new mindset to move forward. even though the Marines still report to the Navy. Haha.. I mean... The Air Force broke away because the leadership felt that the Commanders in the Army were to focused on the ground game that they could not see the best way to add the air assets. You cant see the forest thru the trees scenario.

It is a valid question. I have posted an answer that I hope my fellow Marines and Vets here on Rallypoint feel has at least answered the question in so far as the Marine Corps is concerned.

Cleaned up version of the above, because this is important to me as a Marine, now and forever:
I respectfully submit as an echo, something I heard a senior Marine Corps officer say to a bunch of junior Marine Corps Officers: “The United States does not "need" the Marine Corps. The American People “Want the Marine Corps to exist,” because it consistently does what it says it can do, and when it can no longer do that, than its justification for existing are no more.”

Translation: When the President needs something incredibly difficult, dangerous, and ultimately costly in terms of killed and wounded in action, He (and maybe soon She) calls the Marines, because the Marines get the job done, and Marines don't usually say “it can’t be done.” The Marines, more than any of the other Branch of the Armed Forces still pride themselves on straight talk, no bullshit, whether they are speaking the truth to the President, or to the most junior, newly minted Marine Corps Private who just graduated and walked across the grinder on Graduation Day at MCRD Perris Island or San Diego. When it is "Take a Message to Garcia" time, there is no one better to call than the Marines. This is the Marine Corps' only justification for being. That, and maybe, just maybe, its 240 year history on November 10th, 2015, of Making Men and Women who consistently demonstrate that "uncommon valor, is a common virtue" when Marines fight and win this country's toughest battles. This Nation loves its Winners, and it loves the Marines. That is why we continue to exist as a separate entity from the other Services. When you ask most veterans if they served, they tell you I was in the "Army, Navy or Air Force." When you ask a former Marine if he or she served, they invariably say either “I am a Marine” of “I was a Marine.” The end.

I respectfully submit as an echo something I heard a senior Marine Corps officer say to a bunch of junior Marine Corps Officers: The Nation does not "need" the Marine Corps. The Nation "Wants the Marine Corps to exist, "because it consistently does what it says it can do, and when it can no longer do that, than its justification for existing are no more. Translation: When the President needs something incredibly difficult, dangerous, and ultimately costly in terms of killed and wounded in actionaction, He (and maybe soon She) calls on the Marines, because they get the job done, and they don't usually say it cannot be done. When it is "Take a Message to Garcia" time, there is no one better to call than the Marines. This is the Marine Corps only justification for being. That and its 240 year history on November 10th, 2015, of Making Men and Women of consistently demonstrate an "uncommon valor, a common virtue" and winning this country's toughest battles. The end.

American history has shown that we need four branches because generals have made deadly manpower decisions when the shit hits the fan and had no regard for the other branches when it comes to saving their asses or assets. Also funding tends to be another major conflicting reason. I am sure there are other reasons but these are the first to come to mind