The hidden benefit to the Sample All Layers option (Photoshop CS2/CS3/CS4/CS5)

Many tools in the Tools panel such as the Smudge, Blur, Sharpen, Healing Brush, Spot healing Brush, Clone Stamp and Smudge tools provide you with an option in the tool Options bar, to Sample All Layers. This is useful if you’re retouching a photo that has multiple layers and you want to sample pixels from all the visible layers and not just your active layer.

It’s especially useful when you want to put all of your retouching edits on a separate layer. You can create a new layer above the other layers, select the tool of choice, and then select the Sample All Layers check box. With your new layer active, make all your cloning or other retouching so that all the edits show up on the new layer.

This serves a dual purpose. First, and most obvious, it allows you to sample from your entire document without the need to merge the layers. Second, it shows just how much retouching was necessary for the image in the event that you have to show proof of your extensive efforts to your client.