You have a queue in Rabbit. You have some clients consuming from that queue. If you don’t set a QoS setting at all (basic.qos), then Rabbit will push all the queue’s messages to the clients as fast as the network and the clients will allow. The consumers will balloon in memory as they buffer all the messages in their own RAM. The queue may appear empty if you ask Rabbit, but there may be millions of messages unacknowledged as they sit in the clients, ready for processing by the client application. If you add a new consumer, there are no messages left in the queue to be sent to the new consumer. Messages are just being buffered in the existing clients, and may be there for a long time, even if there are other consumers that become available to process such messages sooner. This is rather sub optimal.

So, the default QoS prefetch setting gives clients an unlimited buffer, and that can result in poor behaviour and performance. But what should you set the QoS prefetch buffer size to? The goal is to keep the consumers saturated with work, but to minimise the client’s buffer size so that more messages stay in Rabbit’s queue and are thus available for new consumers or to just be sent out to consumers as they become free.