The reasons that Goethe included an intervention from A Voice (From Above) are several and have to do with Margarete (Gretchen), Mephistopheles, and Faust.
Margaret has been condemned for the deaths of her mother and baby: the verdict against her was twofold claiming that (1) she was responsible for the "accidental" death of her mother from a "sleeping draught" that was, in fact, a poison (villainously concocted by Mephistopheles) and (2) she was responsible for the accidental or intentional death of her newborn baby by drowning in the river.
We don't know the truth about Margarete's mother. We do know that despite some misgivings on her part about Faust's purity of heart,
MARGARETE: Yet something still seems wrong, to me,
Since you don’t possess Christianity.
Margarete believed in Faust and in the sleeping potion he gave her. She was to administer three drops to her mother in order that her mother would sleep deeply when Faust came to Margarete's bed.
Still, we do know that despite her belief in Faust, she did have one hesitating thought nonetheless: 'I hope that it won’t harm her though!" Yet there is no intimation that the "hurt" she thought of might have been death from poison.
We know less about the baby's death. What we know comes from Margarete's rantings in the dungeon. The way Margarete tells her story in sketchy detail, and we believe her: the baby accidentally fell in the river and she begged others to save the child:
MARGARETE: Quickly! Quickly!
Save my poor baby
Away! Down the ridge,
Now, by the brook,
Over the bridge,
Into the wood,
Left, where the plank is,
There, in the pool.
Seize it now: you!
Thus despite Margarete's liaison with Faust, she is seemingly innocent of crimes against her mother's and baby's lives. Though condemned to death by courts of man and "damned on earth" while she lives, she is still worthy of the hope of forgiveness that Valentine holds out to her with his last breath:
VALENTINE: And, though God may still forgive,
Be damned on earth while you live!
When Margarete commends herself to the mercy of God while in the dungeon,"God of Judgement! To you, myself I give!" Mephistopheles declares her "judged," as though guilty of her crimes, while trying to get Faust out of her prison. It is in retaliation to Mephisto's condemnation that A Voice proclaims Margarete forgiven: "She is saved!"
So the first reason the voice intervenes is to contradict Mephisto and proclaim Margaret pure and forgiven. This leads to the second reason for the intervention: we now know what Margarete's spiritual end will be: salvation and relief from suffering. The third reason relates to Faust's fate at the end of Part II.
Goethe suggested in comments to friends that the only possible end for Faust was redemption, which is very different from the earlier renditions of the Faust legend. It seems that by the early Wiemar years, Faust's redemption was already planned. If so, then the Voice may foreshadow the heavenly intervention from Margarete and angelic choirs Faust experiences at the end of Part II, especially as A Voice (From Above) was added after the original, the Urfaust, was published.