Recently, my friends and I figured out what crop circles are.
First, find a globe.
Second, look at any spot on that globe. Assuming it's a normal globe, you'll see listings of location names. "Europe," "Asia," "United States," "Australia," and so on. Perhaps there are even entries for individual towns, counties, or states.
Third, imagine that instead of our normal letters, your language involved a lot of circular letters, with lines and stuff that go between different circles. Y'know, your letters look like crop circles.
Fourth, pretend you really want a nice, accurate globe of planet earth, with all the important spots labeled, and that you don't want to get any details wrong.
Fifth, you have huge, orbital forcefield beams that can bend crops into the shapes of letters.
So, that's the answer. Crop circles are just giant efforts to label planet earth, for ease of reference by aliens.