We leave the decoration of public space to others. The city, not us, puts up bus stops and rents out billboards on those bus stops. It plants trees and strings them with lights in the winter. In summer, building owners plant flowers and put out flags.
We don't. We walk through, minding our business, occasionally glancing around. Picking up garbage on the street that you yourself did not drop is practically an unbalanced act. Planting a tree on the public way is probably a misdemeanor of some kind.

Most folks, that is. Artists, however, take a different relationship to common space. Enjoying the freedom that creativity brings, and a certain degree of expected boldness, they understand that they can, if they choose, contribute to the beauty of our urban landscape, though the rest of us don't always appreciate it when it takes the form of, oh for instance, graffiti.
In 2012, Pete Dungey, a British artist, created what he called "pothole gardens" -- little plantings in what are normally eyesores and dangers. It was part of a project he named, delightfully, "Subvert the Familiar," an important role for art.
In that same spirit, I noticed and admired this pink and blue striped handle cover that somebody knitted for the Big Belly garbage compactor at the corner of Madison and Wacker Drive. It was not an accidental act. Somebody had to conceive it, create it, and affix it to one of the corporate garbage cans that represent the fire sale sell-off of Chicago's infrastructure. I admired that it was so subtle. It wasn't showy. It was just there.
What's the handle's purpose? Hell if I know. To pad and protect the fingers of those throwing stuff away? To look pretty? To get people like me thinking? Something else? I hope there isn't some common, well-known, prosaic explanation. "Geez, Neil, those are the pink and blue knit handle cozies that Girl Scouts have been putting on garbage can handles for years. Where have you been?"
I noticed the knit cover on Sunday, on my way into the Lyric to see "The Sound of Music," and instantly quizzed its PR staff, wondering if perhaps it was some Austrian decoration—the trashundrecepticalmittenfruppy—they had put out, in some unfathomable cross-promotion of the musical. No, not them, they said.
Update:

I wouldn't mind hearing from a yarn bomber (nice to hear that term given a new meaning) about the appeal, though a very good guess immediately springs to mind. People who knit, who like the physical act of knitting, are faced with the challenge of what to do with the result of their passion. There are only so many loved ones who are willing to accept so many scarves, sweaters, afghans, etc. Thus yarn bombing seems to offer a perfect solution. Takes a lot of knitting to wrap a tree. Plus you get to be artistic and neighborly too.