Origins: These massive vehicles were introduced as part of the resurrected Emperor's campaign of terror six years after the defeat at Endor. Engineer Umak Leth (you know, the mastermind behind the Death Star) developed these bottomless feeders and the Empire unleashed them on Admiral Ackbar's home world of Mon Calamari.
Plausible Non-Draconian Reason to Keep Them Around: Their hyperbolic name aside, these things are actually pretty spectacular at terraforming.
Considerably Less Effective Name: The Pretty Spectacular Terraformer.
Achilles Heel: They are automated devices, controllable by a master signal that can be exploited to shut down.
Continuity Headache: Because of the backwards way the prequels were produced, the events of Episode I take place before the events in Dark Empire, which was published in 1991. That being the case, though, you'd think Palpatine would know better than to hook up his World Devastators to a master computer signal, since it didn't do the Trade Federation any favors during the invasion of Naboo.
Further Reading: The Dark Empire comic series, originally published in 1991-1992 by Dark Horse Comics.
