Meta data can be useful in cases where it's difficult to distinguish objects from each other by looking at their primary attributes like
.name or at their type.
Meta data is especially useful in collision handling where you might need to handle collisions with a, say,
StaticBody2D - but your level is full of
StaticBody2D's - the ground, platforms, walls etc. so to easily find out what type you've collided with you give them a meta data entry
.set_meta('type','wall')) - later on when your player collides with something you look at the meta data for the object and handle the collision accordingly.
... you could also let objects share state via meta data or similar.
So, yeah - use it for whatever you find useful?