It is asked that, if the rhetorical devices in the Qur’an can be traced back to some or the other pre-Islamic poetry or prose, then how can anyone claim that the Qur’an is miraculous, seeing that it is merely borrowing and adapting already existing literary genres? For example, it is said that the rhetorical techniques used in the Qur’an (such as Iltifaat, Tajaanus, Muqaabalah, etc.) were already known and used, and there is nothing miraculous in using what everyone else is using or at least knows about it.
(Thus, it is said that exactly for the very reason that one can make sense of what the Qur’an says by going back to a dictionary and referring back to previous writings in prose and poetry, no person should consider such “traceable material” as being a candidate for miracles).
Answered by Sharif Randhawa (Researcher at Bayyinah Institute, owner of Qur’anic Musings blog), with slight modifications
Very briefly, the point is that the Qur’an far surpasses classical Arabic poetry in its various features, e.g. the precision of its word choice; the beauty of its language; its symmetry, structure, and coherence; and features like iltifat. To use the example of iltifat, there are a few examples here and there, but as Muslim literary critics observed, the level of iltifat in the Qur’an (and its precision, the consistency of its patterns, and its rhetorical effects) totally dwarfed what was existent in Arabic poetry. Indeed this is a very unique feature of the Qur’an.
 A rough definition of this would be that it is a type of digression, sometimes associated with a change in the subject matter, but more so associated with a shift in the person the speech/composition is being addressed to (i.e. changes between the first, second, and third persons within the same passage or phrase).