Service invocation allows the device to invoke a custom HTTP service configured in the basestation. This makes it possible to integrate with external services without needing a client initiated connection to the device. And without needing an HTTPS client implementation on the device: The device uses the already secure connection between device and basestation, the basestation in turn invokes an HTTPS service with the data specified by the device.
Note that the basestation needs to be considered trusted in this scenario as it handles cleartext data from the device (vs. the normal end-to-end encrypted communication between clients and devices). This can of course be remedied by custom encryption between the device and the external service.