The publish-Subscribe pattern also known as Pub/Sub is an indispensable tool for building enterprise-grade .NET applications. Just to refresh your memory, Pub/Sub is a messaging paradigm where the senders of messages (publisher) do not have any knowledge about the intended recipients (subscribers). Moreover, the publisher and subscriber applications do not interact with each other directly but instead depend on a common medium known as a topic. Hence, it's a loosely coupled messaging model.
Now, assume that you have multiple applications of different roles deployed within the same architecture and they need a mechanism to inform/notify each other about certain events. These events could either be transient (due to changes made on the run time) or database events (due to changes in the database). That's exactly where the publish-subscribe design pattern will help you to enable distributed events.