> The first is that because of the nature of message queues, they're either empty or full.
... Wat?
> that is it's a true "fire and forget" kind of request and direct response time is not a concern, then what does the queue gain you over simply posting it to a database table?
Performance is why. The fire and forget aspect is like udp in the sense that you don't need to ensure ordering of messages (packets) or hard persistence to the database. Also, dead letter queues exist for a reason.
Message queues are super useful. The highest performance systems I've seen use the message queue + pool of workers paradigm, as it allows you to better smooth your load (unlike immediate republishing like in sns, which requires hardware available to accept) with minimal guarantees (unlike a write ahead log such as kinesis). The buffer is also great because it allows you a bit more time to scale up both your message queue fleet and worker fleet when you get a load spike.