If we just said 'Yes, you need,' the question "What?" would come up. What do you need? Need is not modifying anything, and due to the lack of the answer to 'what do you need,' 'Yes, you need' would be a sentence fragment.
If we just said 'Yes, you need,' the question "What?" would come up. What do you need? That's true in some/many cases.
But in the context of the test question, what I need (to do) is clear; I need to help you.
My point is that some things are just not said in English, even when the meaning would be clear from the context.
We also don't say 'Yes, I want.' (even if the meaning would be clear)
Need is not modifying anything, and due to the lack of the answer to 'what do you need,' 'Yes, you need' would be a sentence fragment.
Just saying 'yes' would be even less, but we still say it.
Does anyone actually say, "Need I help you?" Where is this used?
I've never heard it.
hehe! It is used in English tests to those Chiese students who are struggling in studying English. And in many cases, what those exercise designers want only is to puzzle the examinees with unpractical grammar questions instead of improving their English.
I, though a so-called English teacher in my country,am always suffering the mental pain of not being able to speak good English. We do not have a satisfactory language environment. And because of the feature of my job, I have to devote most of my efforts to endlessly explaining the boring and stupid grammar points(most of the time, it is really unpractical) to my students, because for many of them the only purpose of learing English is to pass the exam.