Say you're running a SQL transaction and queue a message in SQS. The problem is, this message isn't part of your SQL transaction. So if your transaction fails, the SQS message won't roll back with it, potentially leading to inconsistencies. That's why sometimes it's better to use a queue inside an SQL database, to ensure both your transactions and queue messages are always in sync, and as a bonus, it simplifies the overall architecture and you will have fewer potential points of failure.