Event-Driven Design
Pub/Sub systems are often used to enable "event-driven design", or "event-driven architecture". An event-driven architecture uses events to trigger and communicate between decoupled systems.
Some applications use a "microservices" architecture rather than a "monolithic" one. That just means that the backend of their application has been broken up into many smaller applications that talk to each other.
While it's not always the case, many microservices architectures are event-driven. Instead of one service commanding the other services to do something, they just say "Hey, this thing happened, if you care about it, you can do something".
In the event-driven architecture above, the "auth service" doesn't have to know anything about email verification or billing. It just says, "Hey, a user signed up!" Then the billing and email services listen for that event and do their respective jobs on their own.
Event-driven systems make it so developers have to think about ____ when working on a single part of the system.