The memories! I really liked these type of games when I was a kid. And the twist of not knowing how the finished puzzle looks like adds a hidden aesthetic to the game. It's no longer just trying to figuring out where should everything goes, but also it adds a memory component, because the game lets me know when a square it's in his right position, but since I still need to move it, I need to try to remember each of these positions.
Great job!
PS: It's also nice to see other entries made by Godot. It's refreshing not seeing Unity games. Not that there is something inherently wrong with the engine, but I found awesome to see devs picking up other development tools.