Application of Service Bus

With very much interest I watched on Channel9 Clemens Vasters's series of the Device to Cloud series (http://channel9.msdn.com/Blogs/Subscribe). I'm rather confused as to where/for what purposes using windows Azure Service Bus? Take the example of Clemens's talk about sending metering/telemetry data from small devices (IoT) to a backend. Why would I send the data to a service bus and not just calling a REST service api?

Thanks.

Guy