They're random, and they don't make any sense. That makes them the perfect tool of communication. That's the idea behind crop circles. But let me back up a minute.
It's obvious to any observer that super intelligent aliens have been keeping an eye on Earth for a long time, trying to come up with ways to communicate with us. So far they haven't hit upon a clear method of letting us know they are there, much less that they are more intelligent than we are. But they are brainstorming. Now, the whole idea of brainstorming is to throw ideas out there, no matter how illogical, in the hopes that it may lead the group to come up with something feasible and effective. As Ryan George demonstrates, it wouldn't be logical to assume that super intelligent beings who could travel from distant galaxies to enslave Earthlings can't figure out a better way to tell us about their plans. This video is less than three minutes long; the rest is an ad. -via Geeks Are Sexy