Web applications get more complex over time.
We start with a simple application, create a business on top of it. Start hiring people, and all of a sudden the code does not talk in the same business language. It becomes harder to change.

There are strategies like Domain Driven Design that shows how to put business rules into your code and publish domain events that can be consumed asynchronously by another service.

For example, when a booking is made, we want to send an invoice, reacting to a Booking was confirmed domain event.
By decoupling the code with domain events we have the help of Symfony Messenger.

Symfony messenger allow us to simplify our business allowing to publish and react to those domain events, no matter where we publish them. We can even create specific alarms on some specific events that are important to our business and specific retry strategies.