Sometimes what you witness is “fashion” rather than anything based in physiology or biomechanics. People see it and there’s a “copycat” effect. It makes them feel like they’re doing something.
Just as when you are jumping, there are moments when you can get behind the motion to reinforce your driving aids. But constantly being behind further burdens the hindquarters rather than activating them. And any time you place yourself out of balance, there will be a time lag while you shift your body back into balance.
You will see lots of accomplished international riders who don’t need to sit behind in order to make a good extension. Having said that, if you are on a great big horse that’s hard to sit, sometimes being behind the motion makes it easier to get down into him and not be bounced around. That makes it a practicality even if it’s not especially pretty.