The Levels command, and its cohort the histogram, let you adjust luminance levels on a channel-by-channel basis. The upshot is that you can increase contrast, correct for color cast, and make a bad image good.
An adjustment layer is an independent layer of color adjustment that you can edit any time you like. Plus it affects all layers below it, consumes very little space in memory, and affords the opportunity for selective edits.
Hue/Saturation not only spins colors and intensity values, it lets you modify one color independently of all others. And it does so in such a credible way, you would never know it was there. Perfection.
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