Vergil gives his character, Aeneas, divine lineage to Venus mainly in order to parallel a contemporary situation with his patron, Gaius Iulius Octavianus (Augustus) whose adoptive father, Gaius Iulius Caesar was revered as a god by the senate and people of Rome for his military prowess, thereby giving Augustus similar divine lineage. Some argue that Vergil did this to intentionally promote the image of his patron and on a larger scale, some argue that Vergil's, or Augustus' attempt to glorify Rome by establishing a lineage, as well as a new image of power through the quality of literature itself (remember that Roman literature was looked down upon by the rest of the world, namely Greece) was also propaganda to show off the promising potential of Rome as it emerged from its series of civil wars. Some argue that Aeneas did not prove to be a very good role model for the Romans and therefore would make the Aeneid a poor attempt at propaganda and therefore not propaganda at all. Some can say that the parallel between Aeneas' and Augustus' visions to found a new age in relativity to Aeneas' poor personality traits also would discredit Augustus and therefore would make bad propaganda and ,using the same logic, not propaganda at all. However, I might not see it that way. For the Aeneid, even if briefly, addresses Romulus and Remus. To elaborate, there are a few similarities between them and Augustus as well.
1.) Both envisioned and more or less succeeded in founding eras of progression, embodied by the city of Rome before and now by Augustus' new system of government. That's a plus for Augustus.
2.)Both those eras would come from a dispute, Remus would kill Romulus, and Augustus would emerge from a series of civil wars. This further likens Augustus to two role models in Roman history, you know, the guys who founded the city; I'd say that's another point for Augustus.
3.) Both are associated with the divine. Romulus and Remus with Mars, Augustus with Caesar. Not only this, but Caesar and Mars embody the highly revered aspect of military might that associates itself with the pride of the Roman people, an aspect established by the later of two divines. This further likens Augustus with positive connotation to someone and something already revered in Roman culture and society.
So, now that I think about it, perhaps this could've been propaganda, or at least, it could've logically and rightfully been perceived as good propaganda, assuming people were to notice the secondary and more subtle comparison between Augustus, Vergil's patron, whom he attempted to promote in this case, and the revered figures of Romulus and Remus, the founders of Rome.
That's about it, hope I made sense.