It was, at the time, the scariest thing anywhere near Chernobyl.
This was before that other incident that made Chernobyl famous, so of course back then it looked a little ... fresher ... than it does these days.
But it's holding up pretty well for 40-year-old untended steel.
Given the supervillain-bent of most massive Soviet projects, and, well ... just the look of that crazy damn thing, speculation was rampant: It was a mind-control device, a weather machine, hell, maybe just a giant antenna to pirate Martian pornography. We weren't really on speaking terms with the Soviets, so it's not like we could just ask them.