what is the use case of application event in lightning?

Hi ,

Here’s a simple use case of using an application event to update an attribute in another component.

• A user clicks a button in the notifier component, aeNotifier.cmp.
• The client-side controller for aeNotifier.cmp sets a message in a component event and fires the event.
• The handler component, aeHandler.cmp, handles the fired event.
• The client-side controller for aeHandler.cmp sets an attribute in aeHandler.cmp based on the data sent in the event.

for more detail ,you may refer https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/events_application_example.htm