I think the benefit is that the services don't have to communicate directly with each other. So you have a user's cart that needs tax and shipping calculated. S

I think the benefit is that the services don't have to communicate directly with each other. So you have a user's cart that needs tax and shipping calculated. So you add a message to the queue, and one of the hundreds of nodes in the shipping cluster grabs the message and processes it. In your example, a database storing outgoing messages could be a single point of failure or performance bottleneck. But a distributed message queue would be more fault tolerant. You could have hundreds of sending nodes reading from the outgoing mail queue and sending out emails.