I'd say the following passage provides the direct answer:
The meaning, not the Name I call: for thou
Nor of the Muses nine, nor on the top
Of old Olympus dwell'st, but Heav'nlie borne,
Before the Hills appeerd, or Fountain flow'd,
Thou with Eternal Wisdom didst converse...
Source: Paradise Lost, Book 7, lines 5-9
Milton goes on to describe her as a guide for a journey to the heavens and safe return to earth:
Up led by thee
Into the Heav'n of Heav'ns I have presum'd,
An Earthlie Guest, and drawn Empyreal Aire,
As Urania existed before "the Hills appeard", it makes sense to ask her of pre-historic events:
Say Goddess, what ensu'd when Raphael,
The affable Arch-Angel, had forewarn'd
Adam by dire example to beware...
I don't claim to be an expert on Milton, but he seems to be invoking the primal form of Urania as an something of a personified prime mover (i.e. a natural law, although Newton's Principia wouldn't be published for another 13 years.) Research into Milton's philosophy may provide more insight.
It might also be worth looking at Herodotus' Histories because the work was at some point divided into sections, with Section 7 being named for Urania (The English and Greek texts can be found on Perseus). I don't know if Milton would have had access to this specific text, but he did write in Greek and Latin...
However, simply based on Ovid's profound influence in Shakespeare, I have no doubt Milton knew Ovid intimately. Ovid mentions Urania in his Fasti:
Polyhymnia ended. Clio and Thalia, mistress of the curved lyre, approved her words. Urania took up the tale; all kept silence and not a voice but hers could be heard.
Source: Ovid, Fasti, Book 5, line 53-55 (Frazer trans.) | Latin Text available on Perseus
Ovid has Urania comment on the reverence for the wisdom of elders in past eras:
“Great was of old the reverence for the hoary head, and wrinkled eld was valued at its true worth. Martial exploits and doughty wars were work for youths, who in defence o their own gods kept watch and ward. In strength unequal, and for arms unfit, age often stood the country in good stead by its advice..."
Which seems to fit quite nicely with Milton's note that "[Urania] with Eternal Wisdom didst converse".
