The difficulty in answering your question is in the word "need." Unless you are a professional, you don't need any of this stuff.
That aside, I am a great fan of the 24-120/f4. It is not that heavy. It gives you a terrific range. It's VRII works very well with my D800. And the image quality is terrific. I don't use the 24-70 at all anymore.
So, you don't "need" this, but you will find it extremely convenient and you will probably reduce the number of lenses in your bag and reduce the number of times you switch lenses. It is a good thing, in my mind, to be thinking less about your gear and more about the subject, image, composition, lighting, etc. In other words, the real stuff of the photo rather than messing with your tools.