The original documentation was a little less informative so as to fully describe what an RGB curve does. Does it maximize the color of texture? Or, are the x and y coordinates equivalent to input and output level like in Gimp? And how do the seperate channel's curve work out?
Via RGB Curves node (whether Cycles or Compositor) you can adjust contrast, gamma, gain and also offset adjustments. The displayed curve refer to a given color input brightness. The horizontal axis represents the input values, and the vertical axis represents the output values.
Enable C (Composite) to adjust RGB channels simultaneously. Enable R, G, or B if you want to limit the adjustment to a particular color channel.
Also see the manual:
- Cycles RGB Curves: https://www.blender.org/manual/render/cycles/nodes/types/color/rgb_curves.html
- Composite RGB Curves: https://www.blender.org/manual/compositing/types/color/rgb_curves.html