While Juno is irate with Aeneas, she eventually contributes to the Trojans and Italians uniting.
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Aeneas is fated to ultimately win, so even though Juno makes his life difficult she is unable to prevent his victory. After a lot of fighting between the Latins the Trojans, Jupiter forces her to stop meddling. In return he promises to allow the Latin customs to be adopted by the Trojans.
So basically Juno's "role" is to do nothing... lol.