GIMP Airbrush Tool is amazing. I really hope I learned how to use it sooner. It solves a lot of digital problems I was having.

GIMP Airbrush Uses:

1. You can use it to darken or lighten a color  by spraying a darker or lighter hue on top of the existing color.

2. Good for mixing two colors together.

3. Good for making smooth transitions between two colors.

4. Can be used to make smooth edges or fix a rough outlines and make them smoother.

It has many practical uses. I think you can make a complete digital painting with it without using any other brush tools.