From the SBC Games Blog: "In many situations you need some mechanism how to notify one object in your game with something, that happen in another object of your game. There are several levels how to solve this form tight coupling of objects to loose coupling. If you also want to make parts of your game reusable, then we can say, there are several levels from bad solution to good one from this point of view.
As this is Phaser tutorial, we will not reinvent the wheel, but describe mechanism, that is available in this engine - Phaser.Signal. But first, I will show some bad solutions as this will help to show advantages of signals later in contrast."