It can be very handy to define class_names for your custom game objects, especially if they represent in-game "objects": https://docs.godotengine.org/en/stable/tutorials/scripting/gdscript/static_typing.html#custom-variable-types

This allows you to not only use them as type hints for other game object's properties, but allows you to enter their data inside the editor like so:

This is notably very handy for empowering team members who aren't as comfortable with coding/scripting to participate more fully in implementing design choices and generating content (levels, loot, dialogues, etc).