The ability to send true micro-transactions, securely in real time on extremely lighthardware. There are 'regular usecases' such as paying in real time for streaming or micro-services that this allows, but to see the true potential you have to contemplate what the Internet of Things truly is. in 5 years there'll be 50 + billion connected devices that communicate and cooperate, given the scarcity of resources this ecosystem will need its own economy, up until now no technology solved this problem, but IOTA does.