Years ago, I didn't use layers and simply worked directly on the bit data with color correction, burning, dodging, dust, fixes, etc. Whenever I went too far, I'd have to start from scratch. My images were terminal...
Then I learned the wonders of layers and have never looked back. I can take these work-in-progress files years later and if I don't like what I've done to an image, change any aspect. Because I also label my layers, it gives me an audit trail--I can use that info as a road made to precisely create the same corrections or effect months or years later.
It's a lot like shooting raw, it gives you the option to rework you images later as the software improves and the ability to get even more from your files.
In the overall scheme of things, if you can ace it at image capture because you shoot everything perfectly, then crafting your image through ACR, layers, etc. is a moot point, but for the rest of us, this is just another step toward our digital nirvana.
