Think of an object oriented system. You can have a thread per object at the extreme where each object has its own thread/queue to handle messages. For most cases with synchronous calls you’re not really getting any concurrency.

Maybe it’s hard to imagine now, but in the 80s and 90s there were people that pushed this sort of architecture with a straight face. Even if not this extreme the idea of using threads for componentization rather than a focus on concurrency..which was possibly a side benefit was very much a thing (think COM/CORBA))

Hence why many articles like this and Ousterhout from the 90s, etc saying it was idiotic.