Because, if the dog failed to MARK, the fall, and that is the reason that it needed to be helped or handled, the dog is probably not going mark the repeated fall, if you don't change something.
Just repeating the mark, doesn't teach the dog anything. It will still fail to mark the fall.
I believe that a marked retrieve can be broken down into "sections", and the dog can fail to do any one of those "sections".
If the "section" that the dog failed to do, resulted in it not knowing where the bird is, I don't think that repeating the same thing, is going to produce a different result.
It's kind of like the definition of insanity. Repeating the same thing, over and over again, but expecting a different result. -- Coptor
Maybe, maybe not. It depends.
If a dog fails a 170 yrd land mark due to a factor, let's say, an optical illusion due to slight rolling terrain, and you're confident about your dog's eyes being good and the dog hunts 20 yards short and this is the first time on this terrain, do you handle or have the BB help?
I'd have the BB help, then I'd rerun the mark to see if the dog learned anything about the previous mark: namely, did he/she adjust/calibrate their perception of the distance of the mark? If I don't rerun, I have no way of knowing in that instance whether the dog "gets it". I have to rerun inorder to find out. Now, can I wait a day rerun? Maybe, maybe not; dedends on schedule and access to that spot.
What if I rerun and the dog fails again? Then I shorten the mark so that dog can succeed, then lengthen it back out. If the dog fails again on the long mark, then I stop and ask much more critically, why is he failing on this long mark? It might end up that the mark is broken down into pieces so that parts of the rolling terrain are deal with individually, then slowly pieced back together (much more methodically than simple shortening the mark and lengthening it back out).
Everything in context, everything in moderation.