Why i should event if i can reference to a method through a?

With events you can only subscribe, or publish to them. With delegates you can perform other actions too, such as passing a delegate in another function as callback--although this is no longer a recommended approach and you should consider using async/await for asynchronous tasks and functions.  So do not confuse yourself with the delegates and events, in C# events are the way to go when you want to have a publish-subscribe feature, and this feature is based on the delegates. 

Please see this for more on the topic, https://stackoverflow.com/questions/29155/what-are-the-differences-between-delegates-and-events