I think the most obvious reason is the platypus. I mean, have you ever seen one of those things? Not only do they have webbed feet and are furry, but they also have a beak, which means if you punch it in the face, it's likely not going to hurt it. I think that any planet on which a creature like the platypus can evolve should be granted special privileges, such as moons made of gold and an excessive amount of pretty guys named Nathan.
Earth is also like the shiniest planet ever. If there were some hot 10 billion pound chick floating by, and she caught her reflection upon glancing at Earth, I'm sure she'd say, "Hot damn! My tits are gigantic." I can't speak for other people, but I for one like the idea of living on a planet that might grant me the opportunity to see a pair of 1 billion pound tits.
Our humble planet is also home to the toddler. At first glance, babies don't look like much, but when you're camping in the woods and are in jeopardy of being mauled by a bear, you'll be glad you have something to feed that thing.
Without Earth, Mars would be the third closest planet to the Sun. Were that the case, people would still exist, but they'd all be fat and ugly. Imagine a planet filled to the brim with fat, ugly people. That's a lot of people I'd have to kill.
Finally, Earth is home to numerous technological marvels. While it hasn't been invented yet, I'm sure it's only a matter of time before we create unicorn cyborgs. We'll call them Unicrons and they'll shoot laser beams out of their horns. Not only would they transport us wherever we wanted to go, but we'd totally get laid like all the time if we rode to bars on one of those things.