HSL

Hue adjusts the actual colors of the image.  We can adjust how individual colors behave.  This is extremely useful in landscape work, where you can fine-tune the color of greens and blues, for instance, without affecting browns or oranges. This is great for altering foliage and sky, without adjusting sticks, dirt, etc. It also works well if you have a human subject in your landscape: you end up with nice bright foliage and a nice deep blue sky, but your subject can still have nice, normal, subtle and subdued skin tones.

Saturation slider is a huge help when you want to boost a certain colour over another: I often boost blues and greens on landscapes and sometimes bring yellows, oranges, or reds down in portraits to ease skin tones.

Luminosity is something a drastic.  It's minor, but it's there, and it's one of those little touches that will really seal the deal.

The big thing with the HSL Sliders is to try them and experiment: In many cases, you never know what will look good or not unless you just go ahead and try, and dont be afraid to get drastic.