Queueing theory is a weirdly niche thing to require. I guess almost nobody is a software engineer according to you!
If you have to program ANYTHING that serves requests and has a limited buffer you enter the domain of queueing theory.
Processing internet packets? Queueing theory.
Developing a distributed system? Queueing theory.
Designing software to manage a call center? Queueing theory.
Shipments? Ticketing? Payments? A myriad of other things? Queueing theory.
Fun fact, while posting this reply HN redirected my to a page "Sorry we're not able to serve your request this quickly". Guess what would have helped?
Queueing theory.
What exactly is queuing theory going to tell you about managing a call centre? Why would I need to know some deep theory to be able to stick calls in a FIFO and deal them out to agents?
Are you just proud of having studied queueing theory?