Someone posted a copy of the Bob Cupp grading plan on here once, I believe. Maybe it was somewhere else, but I copied it and have it on my computer.
I believe the swale was for drainage pure and simple. The long slope above the bunkers drained between them, and on to the green. I suspect that as runoff grew over the years, that drainage and the shaded area kept the green from reaching its full potential. From the grading plan, it seems clear that the shelf in the back was also made more pronounced. Probably, at that time, the slope of the green was too pronounced for its increasing speed, and above and below were flattened, leaving the deck in between.