When exactly would you use an adjustment layer that's not at 100% opacity? Seems like a silly thing to do. If the effect didn't turn out exactly right, wouldn't you just re-do the layer from scratch or tweak its slider settings?
That's the way I worked for a long time, and still do in most cases. But there are certain times when it makes sense to create an adjustment layer that goes too far, then dial it back a little by lowering the opacity using the slider in the Layers window. That's a different technique than, say, applying a gradient to a layer mask to darken the sky but not the foreground.
When a photo has a colour cast, the natural adjustment is to do a Levels layer and set the Gray Balance. However, sometimes the photo doesn't really have any gray -- the rocks, street, or whatever is made out of the wrong-coloured material, is under reddish sunset light, and so on. In this case, pick the closest-to-gray colour to apply the Gray Balance, which will swing the whole photo too far towards blue, yellow, or sometimes magenta. Instead of endless re-doing the Gray Balance looking for an elusive speck of pure gray in the photo, lower the opacity of the Levels layer until you find the accurate-looking percentage.
When a subject is a little too dark, but the background is pretty much the right brightness, I like to apply a "spotlight" effect by doing a Levels layer and using a circular gradient for the layer mask, centered on the person's face, their whole body, or whatever subject is too much in shadow. But sometimes, the Levels adjustment that makes the subject look realistic is too bright to blend well with the rest of the picture. In this case, lower the opacity of the Levels layer until the falloff in brightness is imperceptible. You'll still have the "ideal" Levels adjustment setting that you can apply if later you decide to sacrifice the background by brightening the whole picture.