True, you can already change per-app icons in Plasma by manually editing .desktop files. The difference is that kAppIcon turns that into something actually practical:
- Proper graphical interface instead of digging through files
- Easy overview of all your current icon themes
- Easy overview of all your current icon overrides
- Quick per-app icon selection without needing to know where the .desktop file lives
- Support for custom icons and icon packs in a much more convenient way
- Built-in tools like a pixel editor, batch operations, and undo
- Works cleanly on the user level without needing to touch system files
- Squared, Round or Rounded icons are a preference setting
- Very easy to revert because of backups
- Significantly lower barrier for people who just want their desktop to look the way they want, without becoming a config-file expert
So while Plasma technically allows it, kAppIcon makes the whole process fast, visual, and actually enjoyable to use.