What's the difference between using stock and water?
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.