You will see how to desaturate a part of the image leaving other parts untouched. You won't need to select the areas to be desaturated precisely.
We use the plugin version in this tutorial, but you can take the standalone program and do the same to achieve this result.
Now it is a gray scale picture, only the butterfly has kept its colors.
You can experiment with other photos, for example, with a portrait.
Selective desaturation allows you to highlight key points and see the photo in a different way.