Look up around your city and what do you see?

A bird? A plane? No, it's a pair of mangy sneakers dangling from the powerlines.

Shoe-flinging, or shoefiti, has emerged as one of the more inexplicable forms of cultural expression in inner-city Melbourne.
—Lyndal Cairns, "Heels and toes and then a fling," Northcote Leader, January 3, 2007

Kohler says four reasons typically given for shoefiti are fairly well-founded:

—A celebration for a rite of passage, such as a graduation.

—Hazing, such as throwing a freshman's shoes over the line.

—A memorial, to commemorate someone who died near the site.

—A simple prank.

The other two theories are more controversial:

—Shoes signify a place to buy drugs is nearby.

—They're a sign of gang-related activity, such as marking territory or commemorating a gang murder.
—Terry Rombeck, "Laces wild," Journal-World, March 5, 2007

Museum of Hoaxes has a fairly extensive thread on shoefiti under the subject, "Secret Powerline Codes." It incluces lots of comments from people around the world with their own theories on why the shoes are there.
—Ed Kohler, "Secret powerline codes: a couple cops weigh in," shoefiti.com, September 20, 2005