Long ago, in the Before-time, a great evil walked the Earth/roamed among the stars. This evil was eventually defeated/sealed away/stopped in some other fashion, and it stayed that way for thousands/millions of years. But now, the great evil is returning. Our ancestors were awesome; now it's just us. We have to find some way to defeat/destroy/re-seal-up the evil with what we've got right now.
This scenario crops up everywhere. It's in Lord of the Rings (Sauron), The Wheel of Time (The Dark One), Game of Thrones (TBD), Harry Potter (Voldemort), Doctor Who (several uses; mostly Daleks), Babylon 5 (The Shadows), the Abhorsen trilogy (Orannis) and various H.P. Lovecraft works (notably referring to Cthulhu), among many others.
"Sealed Evil in a Can", and give a bunch of other examples. So the best I can do is give my small opinion about why it works so well in these particular genres.
-It instantly creates a sense of menace. Sauron may have an army of orcs, but he doesn't really do anything (in books or movies) other than send the orcs to attack things and gaze menacingly out of the Palantir at Pippin. He's not really all that scary. But if we learn that he once nearly destroyed the world, when he had the Ring... now he's a bit frightening. Likewise the Daleks. In the new series, when we meet them, they could be just the alien bad guy of the week (albeit an astonishingly deadly one). What makes "Dalek" the best episode of Season 1 is their history, and the Doctor's instinctive dread for them. Having what sticks in my mind as a "once and future evil" gives the evil in question instant badass credentials.
-It establishes the heroes' weakness and gives the viewer a sense of risk. Usually, the people who did the defeating/sealing away of the Great Evil were much more powerful than the present day; similarly, the Evil was usually much stronger too. If we know that our modern-day heroes aren't as good as the ones back then, we're less likely to expect them to win just because they're the good guys. We know they'll be overmatched.
Holy hell, I just got trapped in TVTropes for a solid hour. Where was I going with this?
I don't know. Will edit later if it comes back.