> My current thinking is that queues don’t increase average throughput. Instead, they act as buffers that absorb short-term bursts and timing differences between senders and receivers

Absorbing bursts is one purpose for a queue, but often not the only or even most important one. Other reasons include:

Correctness reasons:

* Providing a durable history of requests that were sent

* Creating at-least-once semantics for important (e.g. financial) data

Scaling reasons:

* Allowing a shuffle between producer and consumer to change shard/key affinity (can be done with direct RPCs, but would need extra library support)

* Pipelining variable-cost requests so that consumers can self-rate-limit and stay optimally loaded