A Remote Terminal Unit gathers field measurements at remote or unattended sites — pump stations, pipelines, substations — and forwards them to SCADA, traditionally prized for simplicity and very long unattended life.
A Pi node does the same telemetry job and adds on-device analytics and modern protocols, turning a remote site from merely connected into genuinely smart — while the simplest ultra-low-power telemetry can still favor a purpose-built RTU.