Not a whole lot really. At least not in comparison to actually playing the class/job.

I have learned what a lot of the icons I am seeing mean, especially the tanks. I learned why it is ok for a DPS to pull sometimes. I've learned a lot about how AST works despite me not spending one day playing this job. I feel it should be more from merely observation, but I am always so focused on what the tank is doing. It is a big reason why I will play DPS jobs so I can get a better understanding of what they are doing, or trying to do.