Photoshop is one of the only graphics software programs that allows users to work in layers. Layers are a way of stacking designs, then removing or hiding them to see how your work looks. For example, on a free graphics program such as Microsoft Paint, if you draw a line on an image, it becomes flattened on top of it and you can't move it or add to it. In Photoshop, you can add a layer to your image, draw on top of it, then make any changes required. Layers are similar to a flip book. Imagine your image as the last page of the flip book. Each page of the flip book places something on top of the image, and each page can be deleted, or further designed to enhance the image.
http://www.ehow.com/list_6338237_benefits-using-photoshop_.html
One distinct advantage of layering with Photoshop is that should you want to
change something in your image, say replace the angry clouds with a brilliant
sunrise, then that sort of change is a few mere mouse clicks away, and the rest of
the image lies undisturbed. This can be a huge time saver if for instance you're
working on an advertisement, and you need to test different text or images, or if
you need to change prices or anything else residing in your image. It's no longer
a "from step one" type of project. Simply change the layer you need to, and you're
done!
Working layers in Adobe Photoshop allows you to manipulate your images to your
hearts content without damaging the original image, saves you many hours of
otherwise tedious editing, and can make otherwise unmanageable projects quite
easily within your reach. Learn Photoshop and how to use layers today, and start
seeing your work soar!
http://www.scribd.com/doc/1443942/Working-In-layers-The-Essence-Of-Photoshop
If you need more resources google for "photoshop benefits of using layers".