It's to advertise Vodafone's last mobile customer profile, 1 cent/minute ! (outgoing calls only, incoming calls are always free by default)

When I was an art student in university I did a performance piece called Free Money. I screwed a small wooden box to a stool and put it near a busy pathway at the school. I filled it with change (mostly nickles and pennies) and taped a sign on it that said "free money", and hid in a nearby building with a long telephoto lens and took pics. Most people looked and walked on, some took a few coins, one left a note that said "Thanks- I needed bus fare home." After about three hours two guys dressed in masks and capes ran up and dumped what was left into a bag and ran off with the stool and box, which they brought back after a few minutes. Great fun. If I could have afforded it, I would have used dollar bills- I bet it would have been taken much faster!