I've always been meaning to use the hard mix blend mode to control lighting, but blend modes such as hard mix, difference, and linear light produce extreme results that even the opacity slider can't fix. Recently, I learned about the uses of Photoshop's fill slider. I thought that the fill slider was identical to opacity, with the difference being that fill doesn't affect layer styles. However, the fill slider also affects eight blend modes, some of which seem to perform linear operations: color burn, linear Burn, color dodge, linear dodge, vivid light, linear light, hard mix, and difference. (Source)
While opacity modifies transparency of a blend layer's contents AFTER layer composition operations are performed, fill has a direct impact on the operations themselves. For example, while the hard mix blend mode creates a composite with only eight colors (including black and white), lowering the fill slider reduces the posterizing effect and allows the composite to have more colors, which creates an actual "blend" as opposed to a mess of spray paint. Using fill rather than opacity also takes away the unpredictability of blend modes such as difference and gives users much more control over what happens to the colors and lighting on images. I recognize that Krita stopped being a photo editing app years ago and has little obligation to add more features that seem exclusive to Photoshop. Even so, I'd appreciate a fill parameter for the sake of giving underestimated blend modes the use they deserve.

Postscript: I don't expect anyone to go through the trouble of actually watching the videos from the hyperlinks I included. However, to the rare soul that has the time (probably not the Krita devs; they have a lot to worry about without feature requests getting in their way) to read a few more small paragraphs, the descriptions that go along with those videos offer insight that I can't provide.