Both Levels and the more powerful Curves tools allow one to alter the pixel values of an image - but it does this for ALL pixels with the same values. If we wanted to selectively change some areas, but not others, we would have to use Layer Masks to mask out the areas we did not want to be changed. This may not be easy, and can be time consuming, to mask out those bits exactly.
But lets see what happens if we use a Curve instead of a Channel blend on this image.
I've over done the adjustment to make the difference between these methods really apparent. It doesn't look too bad (at this small size) - but we have done lots of terrible things to this image! Let me show you...
First one would probably have done the Curve on an Adjustment layer (so we could change our minds later). It would not have occurred to us to change the Blend mode, of that layer, and we would have just left it at Normal. Well these next two images show the differences made (using my SeeChanges action), and compare the Normal and Luminosity blend modes. Remember when looking at these exaggerated false images, that Black means no change and White or colour means a lot.
Hopefully I've convinced you that although Curves, and Levels, are great tools, sometimes you can achieve results that would be impossible without using masks, just by a quick Channel blend. The downside of this technique is that you must be able to 'see' what is in the image - and that takes practise.
Another point, well worth mentioning, is that it is the ratio of the changes that you make, in either tone or colour, that will effect how the viewer perceives the image and thus how you can you can draw their eye to different areas and better tell your story - isn't post-processing fun!