When many parts of a system need to react to the same event, wiring each sender directly to each receiver becomes a tangle. In the publish-subscribe pattern, a sender simply announces that something happened, and any part that cares subscribes to receive such announcements, with neither needing to know about the other.
This loose coupling makes systems easier to extend, since a new listener can be added without touching the sender. The trade-off is that the flow of cause and effect becomes less obvious, which can make a sprawling system harder to trace when something goes wrong.