Hi Pete,
I know this is Bob’s show, but I know he won’t mind if I chime in once or twice. Around here we use keyways to keep the bottom of the foundation wall from sliding off of the footing. I am sure the rebar will work just fine, but as usual, the almighty dollar prevails and a keyway is cheaper than rebar. That is the groove that is shown in Bob’s picture. Usually they are not that clean.
Bob, you asked why they don’t pour them at the same time. You would have to box the top of the footing to keep the concrete from coming out since the walls are thinner than the footing. In doing so, air would be trapped and there is no way to confirm that the concrete fully filled the void. Another reason is removing forms from inside corners is a pain in the *&^.