Plenty. Before I start though, I’m going to point out, “destroy the earth” is a very cheap narrative device. It’s an attempt to generate tension with the reader by saying, “hey, you live on Earth, you care about living, hey, I’m going to try to make you care by blowing up something you know.” This also runs under the surface of the “New York/Paris/Wherever gets blown up,” and “terrorists will detonate a nuclear weapon,” narrative.
That said, you can uses them. 24 managed to crank out 9 or 10 seasons of threatening to blow up cities or otherwise annihilate western civilization on a remarkably short schedule. But it is a cheap device, and it’s entirely possible to lose your readers on this.
Blowing the place up is probably more interesting than the threat to do so, and can catch your readers off balance. “Of course your heroes are going to save the day, that’s what they… oh…” Just make sure you’ve got someplace to go, once you cross that line.
So, here’s some random reasons:
It was in the way: This might be to create the Douglas Adams Memorial Hyperspace Overpass, or it could be someone just wants to shatter the planet, so the mineral wealth is easier to mine in an asteroid field. It could be the planet is in the path of an interstellar super weapon.
An Accident: This could be your villain is just that bumbling, however it could also be that they really don’t care.
The aliens in Roadside Picnic come to mind as an example of the latter, along with nearly all of the aliens in Lovecraft’s Cthulhu mythos. These are beings which barely even perceive humans, or view the as stray pests.
This is also possible as an unintended outcome of some technological development. For example, during the Manhattan Project there was reportedly a fear that detonating a nuclear device inside the atmosphere would result in a chain reaction, burning off the atmosphere.
A similar possibility would be the danger of an engineered bio-weapon getting into the wild and annihilating the population.
Prophesied: I’m throwing this one in here because it’s legitimate, but I’m going to start with a warning: Writing prophecies can be very tricky. They run the real risk of being horribly cliche in their own right. As a writer it can be very tempting to say, “well, yeah, that’s how it will play out,” because you’re the one controlling the future of your setting.
With that in mind, it’s entirely possible to have someone who is trying to destroy the world because they want to summon some apocalyptic horror, or usher in a new golden age for their sect in the aftermath. This could be real, or they could be cribbing off a 300 year old fast food menu, and drawing their conclusions on how to bring about a new era that way.
We Can’t Let The Reds Win:
Scorched Earth is adult version of saying, “if I can’t have it, no one can.” It’s entirely possible to have a villain who would rather see the world burn than in the hands of your heroes, or some third faction.
This could be some variation of WWIII, or it could be a lone crazy falsifying a nuclear retaliation when none is called for.
An Object Lesson: As with prophecies, this one can be tricky to handle. But, if your villain is threatening to blow up the planet to ensure fealty, sometimes it’s just going to be more efficient to get it over with.
If you’re a comically exaggerated super villain: Stop telling me how you’re going to blow up the planet to “send a message” and just do it.
Obviously, you can mix and match these as you see fit. They’re not mutually exclusive. You can have a mix of the above in play. This also certainly isn’t a comprehensive list, so you don’t need to feel constrained by the examples above.
Your villain wants to destroy the world, obviously they feel they’re getting something out of it. You just need to ask yourself, “what is it?”