I’ve been wrestling with this concept for a while now and now I think I understand what I/other people are asking for regarding the Edit > Adjust Hue/Saturation control.
By default, this tool doesn’t adjust hue/saturation of grayscale colors. This is how it works in Photoshop as well (yes, I’m aware it’s rather unfair to compare Aseprite to Photoshop but bear with me). In Photoshop, if you want to add color to grayscale shades, you can check the “Colorize” box. This causes all the colors being currently edited to assume the same hue/saturation values (Absolute instead of Relative) while Lightness remains relative: creating many various shades of the same hue/saturation.
This feature is super helpful for taking grayscale shades (metals, clothing, weapons, stones, walls, etc.) and changing them to shades of color. It’s so common in pixel art to want to create a new set of color shades from grayscale values that it’d definitely make for a helpful feature.