What are the reasons for adopting a balanced riding position?

Riders who have a proper seat are less likely to attempt to use the reins for support.

Riding on the longe with no reins and no irons is one way to acquire good control of the seat and the legs. Miraculously it even improves the hands. Unfortunately, riding on the longe requires an assistant or instructor who is capable of working the horse properly on the longs.

Proper balance helps the horse and rider cope with the effects of gravity.

Without balance you wouldn't be riding, you'd be falling.

In the absence of balance, gravity takes over.

You don't fall of as much

It's easier for the horse to move under you if you stay in the correct posture for the movement