Explanation:
The traditional use of the pull model, where the client requests data from the network does not scale when what you want is near real-time data. Moreover, in some use cases, there is the need to be notified only when some data changes, like interfaces status, protocol neighbors change etc.
Model-Driven Telemetry is a new approach for network monitoring in which data is streamed from network devices continuously using a push model and provides near real-time access to operational statistics.
Applications can subscribe to specific data items they need, by using standard-based YANG data models over NETCONF-YANG. Cisco IOS XE streaming telemetry allows to push data off of the device to an external collector at a much higher frequency, more efficiently, as well as data on-change streaming.
Reference: https://developer.cisco.com/docs/ios-xe/#!streaming-telemetry-quick-start-guide