I use masks in Photoshop all the time. Take a look at the layers and masks I used to process this photograph from Valley of Fire State Park in Nevada. You can see that I used a mask on all but my background layer.
Here are the images I started with. These are actually a single exposure processed twice – once for the foreground, and once for the sky.
In this case, the line between the sky and the foreground was well defined. I used the color selection tool to select the brighter areas in the sky in the lighter image, and then created a mask from that selection on the darker layer. The mask allows us to see the sky from the top layer, and the foreground from the lower layer. After creating that simple mask, I zoomed in and checked it for accuracy. I needed to subtract some areas of the foreground from the mask, and add some areas in the sky for a cleaner mask. I blurred the mask very slightly to smooth the transition between sky and foreground, and used the precision mode on my Intuos 5 tablet from Wacom to take care of a few small areas that didn’t blend perfectly.
I used a much simpler mask to bring down the brightness of the sky with a curves adjustment layer. Does this mask remind you of a graduated neutral density filter? The effect is the same.
I also felt that the white areas in the foreground were getting a bit lost. I brightened them up a bit with a curves adjustment layer and a mask created from the color selection tool. I feathered my selection very slightly – by about 1 pixel.
When I make adjustments to an image, I often want to target those a adjustments to a specific area or color range. Masks let me be very precise with my adjustments.
There are infinite ways to use masks in Photoshop. You are limited only by your own imagination. These are just a very few tips for making your workflow easier. Please feel free to add your own tips and suggestions in the comments.