I came first across queuing theory in the context of managing software development processes. it looked at the flow of bug fixes and feature requests through the hands and brains of people out to users, and how task switches of those doing the work and task and context switches of teams and set-up times for steps like testing or doing a single change how all these timings and little queues at each step add up and define team productivity.

"The Principles of Product Development Flow" by Donald Reinertsen.

it made me realize queuing theory as a framework for "agile" or "lean" workflows, pull (kanban) or push (scrum), without ever using any of these buzzwords.