Being able to control when and how we fall in an emergency (not simply because we are slightly uncomfortable) is one of the best tools we can give both ourselves and our horses and the time to learn this is not when the horse is collapsing underneath us or threatening to kill us in some other manner. I don't care how experienced you are, you simply never can know when a horse is going to fall out from under you or have a bee sting it under the girth or whatever. Saying that if you only ride horses at your level, you will never have to worry about this is about as naive as saying that if your horse is perfectly trained, you will never need to wear a helmet because they will never do anything to hurt you. Both attitudes are inviting severe injury or worse.
If a horse is going down and possibly over, you don't want to go down with it. Knowing how to dismount safely in this type of emergency situation kept me from being crushed when the horse I was on tripped out from under me and went down in a head over heels rolling motion that would probably have resulted in my leg being crushed and my rips broken at the very least. As it was, because I had been trained and had the wherewithal to fling myself off and away from the horse as I felt what was happening, all I got was a bruise on my hip. This dismount/fall had nothing to do with bolting, or spooking or being overfaced. I was on a well trained horse that simply tripped as I cued a transition from trot to canter on a trail.
Knowing how to get off a horse safely if you have to is invaluable.
A. Because it keeps you from getting crushed/hit by a semi/etc, and
B/ Because by requiring you to learn how to dismount/fall safely, it takes the fear of falling away. You build instincts that will kick in without having to think, whether you are choosing to dismount in an emergency or you find yourself falling despite all best efforts - maybe say like in a rotational fall on a x-country course. If you have developed the instict to land on your side, push/roll away from the horse, drop the reins, etc., by practicing in a safe, controlled environment, you don't have to think in an emergency, you just do. And you don't fear falling when it comes, you just do what you have to do to get through it safely and get back where you belong - in the saddle.