The reason why they sit into a fence is so that they can use their seat to push the horse forward or, by bracing their back gather the horse to shorten a stride.
If you watch you will also see that they come back into the saddle as the horse is coming down from the apex of the fence, again this is so that their seat is ready to drive or collect a horse.
With sitting into the saddle into and on landing from a fence, it is also far, far safer for the rider because they are not in front of the movement so, if the horse does refuse, run out or peck on landing, the rider is not already halfway out the saddle as you are by remaining in two point right through the approach, take off, jump and landing.
I have never understood the US Hunter Jumper classes where nearly all keep in the two point all the time. If any of the riders ever followed Foxhounds then they would not stand an earthly of staying on a horse of there was a drop fence or the horse made a mistake.