Your graphics application lets you do all you want on the colors of an image. But why doesn't it allow the same type of flexibility with the transparency? Surely being able to decide whether a color can actually be seen at all is at least as important as being able to decide what that color should look like.
AlphaToys is a set of 16 filters that let you work on the transparency of images; the Alpha channel.
Sure, your graphics application will probably allow you to change parts of the transparency in your image using an eraser tool, and you can probably delete selections. But can you quickly convert from color information to transparency. Can you easily blur the transparency? Can you conveniently sharpen the transparent edges without having to resort to a longwinded process? And if you do, can you go back and tweak it afterwards without having to go through all those steps again?