I think the key is that if you plant that foot, your body naturally pivots into the full back position. If you don't, you have to pivot (whirl) on that foot which can cause you to get out of balance or have timing issues.
You've got a nice solid base on that foot, and the work of turning is already done. Plus, it helps your aim in that it makes for an easy body checkpoint. Instead of focusing on a target 300 feet away, you focus on your step. You step consistently, follow through the same and every time you plant that foot with the heel in the right place you should get a good throw.
It's kinda like bowling-- you mark your start place, walk straight forward, and aim for a mark on the lane instead of aiming at the headpin. You hit your mark, you will get a strike.
I wish I could do a huge reachback like Stokely, but my timing going forward isn't up to snuff. The loss in accuracy and timing caused by a long reachback offset the advantage. I can't use the foot to aim, but the trade-off is I'm not turning around as much so I can keep my eye on the target the whole time. I think it also cuts down my distance, though.
But I do make sure l plant the foot so I am turned around as far as I'm gonna go and ready to make the release motion, instead of having to pivot on it.