Why slow ferment after shaping? Many people say you have to slow ferment your bread after shaping to give it better flavor, but I can never understand that.  Why not just slow ferment the whole batch then shape it when it is done fermenting? 

This way is way safer as you can't ruin the proofing if you forget it a bit longer than intended and it has been giving me the desirable results I am after. 

Are there any advantages in slow fermenting individual breads instead of the whole batch?

I am talking about crusted white bread. After the slow fermentation for the whole batch, I will just shape the bread, put it in a dutch oven and let it proof for 10-20 minutes, slash it then pop it in the oven to bake. For the other method (which I don't use) they just get it out of the fridge, slash it and pop it into the oven.
Your first batch is used for building gluten strength in the dough for shaping and giving the yeast time to wake up and do their job. After shaping, you are letting the dough rise to its final form while still adding flavor from the yeast.