Student or Learner
The authors are talking about grammatical devices that help the user to select which element of the sentence will be highlighted (cleft sentence, fronting, extraposition).
At the end of the chapter comes the following sentence:
We have merely hinted at the communicative value of these apparently unneeded constructions, and at the stylistic considerations which may lead users of English to select them.
(A comprehensive grammar of the English language, pg. 89)
Have I understood the meaning of the word in bold?
We have merely hinted at the communicative value of these, as it might seem at first glance, unneeded constructions...
NOT A TEACHER
(1) I think that your interpretation is spot-on. (As our British friends say.)
(2) Those constructions only seem to be unneeded.
(3) And this book has only given a hint of their actual communicative value.
(4) For example, which sentence in a court of law would probably have the
greater "communicative value"?
(a) Tom did it.
(b) It was Tom who did it!
(I am sure that we agree on which one would probably impress the judge more.)