"Or you could simply use the eraser tool to erase parts of the original layer and merger the layers when you're done." True, but with a mask you can undo/redo edits. If you delete from the duplicated layer directly you'd have recreate it if you wanted to back out or try something different. Shrug. Also imagist, thanks for reminding me of the history brush, I tend to overlook that feature and it recently helped me out a lot :)