The idea had already been initiated in 2007, but the first release was in 2010 due to typical geeky laziness.
Flattr was founded to help people share money, not just content. Before Flattr, the only reasonable way to donate was Paypal but the transaction cost was (and still is) too high. Visitors would ignore the option. Sending just a small sum has always been a pain in the ass. Flattr is changing this.