Jupiter, or Zeus as his Greek pals call him, is the most powerful guy on Mount Olympus: he's the king of the gods. Everybody—gods and mortals alike—has to do what he says. Cross Zeus the wrong way, and you might just get a lightning bolt in the face—or worse. (Don't believe us? Just ask Prometheus and Pandora.) It's Zeus' job to make sure that the good are rewarded and the evil are punished.
He sure doesn't disappoint. Jupiter shows up for Posthumus in his time of need: the poor guy's ghost fam make one last ditch plea on behalf of Posthumus, and Jupiter actually came through for them. His appearance might seem a bit silly—not to mention random—but it does show us that we're rooting for the right guy. Jupiter would probably not waste his time helping out a bad guy, so we can be pretty sure that Posthumus is A-Okay.
Jupiter also gives us a link to the setting of the play in super-ancient, pre-Christian Britain. Sure, Cymbeline was actually a king who ruled in those way-back days, but Jupiter's presence tells us that people in this play are still worshiping the gods (as opposed to a specific Christian God). It's not that important that these ancient Brits wouldn't actually be worshipping ancient Greco-Roman gods; Jupiter's just there to show us that this stuff all happened in a really long time ago, basically in fairy-tale land.