I could start by saying that there are some special effects that can be easily achieved with an airbrush that are very tedious in when done with a traditional paint brush. There is a much simpler reason and that is airbrush gives you the ability to paint on almost any surface. For instance even though I have painted a wrap around mural on an 18 wheeler truck using traditional paintbrushes and automotive paint it is much easier to do with an airbrush. Uneven surfaces, concrete block for example, are much easier with the airbrush and it is easier to maintain the smooth no brush stroke look on metal. I also enjoy the way it almost forces you to think of color mixing in the eye. Air brush can spray transparent layers especially on paper. But when painting on metal you are generally spraying dots of color. If you do try it I want to remind you that acrylic colors while listed as non toxic often have toxic solvents and you need to learn and observe all the safety precautions.
BECAUSE OF MY EYE CONDITION I NO LONGER AIRBRUSH.