Since GIMP allows to new layers to differ in size from the canvas size, does it matter? Whether a layer of different size from the canvas has uses or hinders.
Not only in gimp but other programs aswell becouse at the end you need an output file in a flat format (like a png or jpg).
One layer could be a little plane and the background a big sky. Of course the layer of the plane has one size, but the important thing is the whole scene.
You can also have some floating images outside the artboard. You can use that space fo place some temporary elements, but at the end they must be croped when exporting the final image.
If you look out at the Introduction section of the GIMP main web-site. Also try looking out how to change the canvas size just like in photosop https://docs.gimp.org/en/gimp-image-combining.html
Thank You for question.