At the beginning of Aeneid 6, Virgil describes Aeneas’ ascent to the acropolis at Cumae with the Sibyl, so they can sacrifice to Apollo in his temple there.
Aeneas becomes lost in thought when he sees the pictures on the door of Daedalus and his adventures. The Sibyl breaks his reverie and says, “non hoc ista sibi tempus specatcula poscit; nunc grege de intacto septem mactare iuvencos / praestiterit, totidem lectas de more bidentes.” (6. 37-39). Now is not the time to gape at spectacles like that.
It is almost dream-like, how Virgil/Aeneas envisions the whole story of Daedalus and the Minotaur, not without some emotion (6. 21). Virgil goes into some detail for at least 15 lines, reaching further and further back into the history contained in the myth.
I think this is meant to parallel the experience that the readers themselves will have in the Underworld with Aeneas. At first, Aeneas becomes lost in thought and the Sibyl must keep him on task and remind him why they are there. While the reflection is not in of itself a bad thing, one must not get so lost in thought that they lose sight of why they are there.
The ecphrasis may not seem terribly relevant to the story, but it is if you consider that it is possibly meant to foreshadow and parallel.
Aeneas is paralleling the Roman readers and foreshadowing their experience when they are reading the catalog of famous Romans and reflecting on the history. The difference is that for Aeneas it is the future, while it is the distant past for the audience. For Aeneas, the story of Daedalus is in the distant past and he slips into reverie thinking about it.
Just as a Roman reader might slip into reverie thinking about the past of Rome and all her heroes, Virgil brings us out of the Underworld again and into a completely different world. The readers emerge into a mysterious, pre-Roman world filled with violence.
Jarred out of his reverie, Aeneas goes with the Sibyl to sacrifice and eventually finds himself in the world of the shades.
One might say that the Roman reader never really emerges out of the Underworld until after the poem ends, since for the Roman (or modern reader) they remain in the world of the shades, in a sense. It’s all still in their past, among people who, if they ever really existed, are still shades to them.
After a reflection on Roman history (though in the future) Aeneas emerges into his own timeline. Likewise, the contemporary reader remains in the world of the past and will emerge into their own contemporary timeline when they have finished the poem.
After reflecting upon the past history of Rome, the reader may have a better sense of their own identity, as Aeneas must have when he emerged out of the Underworld. Armed with that new knowledge, Aeneas could proceed in his destiny.
In a sense, I believe that Virgil is pointing his readers to reflect upon their own experience with the poem, and this becomes fully apparent in Book 6, first foreshadowed by Aeneas’ reverie about Daedalus (and also by the rude awakening he receives from the Sibyl). Likewise, the reader will receive a similar awakening when the poem ends. In both cases, they ought to have emerged with a heightened sense of their identity.