Shadow Boxing is one of the best exercises you can do to increase your boxing fundamentals and overall boxing game. The problem is most people don't know how to do it properly or have any motivation to do it.
There are many benefits to shadow boxing. Unlike the heavy bag if done properly there is alot of footwork involved just like there would be in an actual fight. Also while shadow boxing your punches aren't hitting a target which forces you to maintain balance and by punching the air you get a feel for what it would be like in a boxing match.