Hi Herosea.
There was always an implicit "background layer: that artwork was being drawn against.
But people (when copying, saving images) wanted the ability to preserve the artwork's transparency and to control whether the background layer is included or not.
Now that we show it, it's possible to control whether it's copied or not by hiding its visibility when copying/saving images, which is useful for combining it in other artwork. If the eye icon with a slash through it and transparency checkerboard is visible (double tap on the layer to do this), artwork will export with transparency.
In addition, some of the Layer Blend Modes require a background to matte against to appear properly.
I hope that helps. Please let me know if I can answer any other questions about it for you.
Sue.