Decouple the timing of the request from the timing of the work, so that the user-facing path completes quickly and the downstream processing runs at its own pace.

Eliminate the database write that would otherwise be required for each new order, because the queue itself acts as the system of record for every order processed by the system.

Guarantee that no order is ever lost, because messages held in a queue are protected against any kind of consumer-side or producer-side failure that may occur during processing.

Reduce the number of services involved in the order flow, because the queue absorbs the responsibilities that downstream services would otherwise perform.