when a child sending a signal to a parent (since using get_parent() is a bad practice from what i read) or when a node needed to broadcast a signal to whole node about something, it will come useful
notice that the signal is a built-in signal made for Area2d node, you also can create a custom signal depending on what you need
Also, take a look at Global EventBus for simpler signaling:
https://gdscript.com/solutions/signals-godot/#:~:text=the%20script%20file.-,Global%20Event%20Bus,-In%20GoDot%20we