Network stream drivers are a layer between various parts of rsyslogd (e.g. the imtcp module) and the transport layer. They provide sequenced delivery, authentication and confidentiality to the upper layers. Drivers implement different capabilities.
Users need to know about netstream drivers because they need to configure the proper driver, and proper driver properties, to achieve desired results (e.g. a Encrypting Syslog Traffic with TLS (SSL) [short version]).