This piece is called a "stabile" in contrast to moving "mobile" (like the things that hang from babies' cribs) pieces popular at the time. The artist, Alexander Calder, wanted to break up the right-angles of all the steel and glass structures nearby in creating a more fluid, flowing form, that would be clearly anchored in place. Additionally, he wanted to use more than just the black and grays of the buildings and chose a bright color, now known as "Calder Red".
You can also walk all around and under the piece, getting a sense of scale and seeing unique perspectives.
Finally, though the piece was officially unveiled on the plaza in October of 1974, Calder clearly "signed" the piece and dated 1973, which is the year he completed it. I also find it interesting that, to my eye, his letter "C" is first in his signature.
I'm not a huge art buff, but recognized the name when I saw it. I'm pretty sure if you are considering the purchase of any of his works now, you already have your name on a museum and can afford whatever you desire...though the current owners of his works may not be willing to sell. I certainly doubt the city of Chicago would.