One thing level curves are incredibly useful for is identifying even the smallest dust goobers on your sensor. To do this for a blue sky background, for example, activate the tone Curve adjustment and place a handle about 1/3 of the way up the diagonal line and pull it upwards to appoint slightly above the mid horizontal gridline. Next, place another handle two thirds of the way up the diagonal line, and pull downwards to be low the one quarter horizontal grid line. The result will be an orange sky with despots standing out as bright white objects against this background. Using the healing tool, you can very easily zap all of these, and paste them to all images in your workflow.
Backgrounds of other colors require different manipulations of the curve, but all are basically two handled operations, and simply playing with it while viewing the preview will reveal a combination that highlight your dust spots. Once you have zapped all of your despots, simply turn off the tone curves adjustment and your image will return to normal.
Hope that this helps.
P.S. I have come to DxO from Apple’s Aperture, so I agree let the tone curve superimposed over a histogram would be helpful because you can see the effect that your adjustment is having on the appearance of the image as a function of color channels all in one place. It’s not really a big deal, but I had gotten quite used to it.