What you have stated is exactly right. However, the benefit depends upon what you are trying to do. Generally,
you would want to practice on keeping good technique, while improving muscle memory and movement. You are trying to visualize actually attacking and defending against an imaginary opponent.
Unlike kata, this is not prearranged movements, but very organic and can be focused on the use of a few or several techniques that you wish to get down or improve upon, before actually using them against a responsive and resisting opponent.
Shadow boxing can be as hard or as light as you like, aerobically speaking. But in all that you practice, you must keep proper form and try to evaluate the techniques that you are doing.
This, in my opinion, is one of the fundamental solo excercises that you can do to improve your performance.