What's the difference between using stock and water? I realise this sounds like an odd question.  However, to use an example, when I make cottage pie, a lot of recipes suggest using beef stock.  Okay, so as I understand making beef stock, you basically take some scrag ends of beef (bones, etc), and boil them.  So what's the difference between doing that, and adding the water directly to the cottage pie when you make it?
While the liquid that you end up with in your pie will be similar to beef stock in that it has had beef cooked in it, the difference is that the flavor compounds will have cooked out of the beef already in your dish. This means it's basically just diluting the flavor of your beef.

If you start with stock on the other hand, there is beef flavor is in the stock itself so it's actually adding more flavor compounds to your dish rather than just spreading around the ones already there.