So if we should always apply tonal correction using adjustment layers, are the Curves and Levels options under the Image > Adjustments menu redundant? While you may never need them for tonal or color corrections to an image itself, they are still useful when adjusting the gray values of a layer mask or channel mask (see "Choking a Mask" in Chapter 4, "Layer Masks," and "Forcing Contrast" in Chapter 6, "Channel Masking")since you can't apply adjustment layers to either layer masks or channel masks.
Figure 5.13. The original image (example A). A Curves adjustment layer is applied to lighten the foreground; a gradient layer mask restricts the effect of the adjustment layer to the bottom half of the image (examples B and C). Using Curves on the layer mask affects the transition from black (concealed) to white (revealed) (example D).