In the example on the top, the event gets dispatched when the node it’s binded to gets triggered. This specific node you got here logically searches for an event to call due to it’s nature, that’s why it has that pin named “callback”. (I was gonna correct this but wanted to keep my last sentence to show the wrong way of thinking, it’s not actually that node that calls the event) About event dispatchers, their main purpose is to be able to maintain communication between multiple sources.
If you want to dive deeper, here’s a topic with many detailed explanations in it, specifically this single post right here sums it all up: Why use an event dispatcher over calling a function? - #4 by CriErr