Disclaimer: the images below are not models of perfect curves, but just rough examples of some settings i find helpful.
Also, those are not any close to all possible magic things you can do with curves. Explore ^^
Add contrast to a light Image
Lets say you have a light image with few contrast.
Moving down the dark tones and maybe midtones nodes of the Overall curve will add contrast to it:
Make a dark image brighter
We may also want to do it the other way around: make a very dark image a bit brighter.
Had to go a bit crazy to lighten this up:
green Lights and purple shadows
You can also use the Green curve to add contrast:
Now add some green on highlights and remove some green on shadows to make it a bit more dramatic:
Blue curve for warm lights
Sometimes, you may want some "warmer" highlights.
Use Blue curve to remove some blue out of the highlights: