Why first one is correct?
This essay will discuss both advantages and disadvantages.
This essay will be discussing both advantages and disadvantages.
Is it because it is the only semantically correct one here?
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Sign up to join this communityBoth sentences are grammatically correct, but they mean different things.
The first sentence could very properly be part of the essay itself - almost certainly in the first paragraph - telling the reader what to expect from the essay that they are reading now.
The second sentence refers to an essay that is not yet in the reader's hand. It suggests that an essay yet to be seen by the reader, and maybe yet to be written, will be covering certain topics.