I think the flashing is done just to maintain the integrity of the underlying dirt so you dont have issues of cave in or rebar moving. I have seen it done on some builds with no issue. I will let the construction guys comment as I am certainly not an expert on the matter but just giving my 2c
We had this done (the gunite company actually did it without us even asking) on our pool when a hurricane was approaching. They put a very light skim coat on the dirt to lock it into place and prevent cavein's or collapses.