The delegate pattern, as you call it, is know as the event handler mechanism in other languages. I think it was C# that introduced the term "delegate" for me. As such it simply is an implementation of the observer pattern with a major restriction: only one listener at a time.

The disadvantage of event handlers or delegates is obvious: only one observer.

The advantage is not so obvious: performance. With an observer pattern you can add many observers. When an event occurs that the observers need to be notified about, you will need to enumerate the observers and send a notification to each. This can quickly bog down any observed instance, especially when the number of events that require notification is significant as well.

The delegate pattern, as I understand you, is know as the event handler mechanism in other languages, for example Delphi. As such it simply is an implementation of the observer pattern with a major restriction: only one listener at a time.

The disadvantage of event handlers or delegates is obvious: only one observer.

The advantage is not so obvious: performance. With an observer pattern you can add many observers. When an event occurs that the observers need to be notified about, you will need to enumerate the observers and send a notification to each. This can quickly bog down any observed instance, especially when the number of events that require notification is significant as well.