Which of the following is correct?
Your grammar is so good. Why do you need a grammar book for?
or
Your grammar is so good. What do you need a grammar book for?
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Sign up to join this communityWhich of the following is correct?
Your grammar is so good. Why do you need a grammar book for?
or
Your grammar is so good. What do you need a grammar book for?
What for is a set phrase meaning:
for what purpose or reason
(Merriam Webster). Basically it means the same thing as why.
You can either say:
What do you need a grammar book for?
or
Why do you need a grammar book?
Aside from JR's example in the comments, you can find more example sentences at:
LDOCE, Cambridge dictionary or Macmillan.
To make the usage of what ... for? clearer, you might ask yourself how would you answer such a question. For instance:
Why did you stop at Tom's office? (What did you stop at Tom's office for?)
You can reply: I stopped at his office to have a chat about our marketing strategy, but you can also say:
I stopped at his office for a chat about our marketing strategy.