I am going to answer with what is probably the wrong answer. That will spur the experts to correct me.
A clay bar is similar to polishing, it removes stuff. However it doesn't remove paint as much as polish, if at all. So theoretically it does the job of removing stuff without removing paint/clear.
In my mind that is waisted effort. I mentioned earlier what makes paint shine is being clean AND flat. A flat surface reflects light better. So a good polish with a light abrasive cleans and flattens.