All of the pictures are great, but only Mike Cirba's captures or depicts the inverted bunker concept Tom MacWood references.
These bunkers still exist on holes at GCGC, such as # 1,
# 3 and # 10
I can't speak as to why he inserted them, but I can speak to the effect they might have on a golfer.
I believe that they create a deception.
That they make a hole and its features appear more difficult.
Off the tee at # 3 they are clearly visible and perhaps are there for the purpose of intimidation, as they are rather short off the tee, and it's almost impossible for a ball to come to rest on them.
They also exist short of the green on # 3, hiding the land between them and the green along with two DEEP bunkers that are also short of the green. Again, the chances of a ball coming to rest on them is remote, so their purpose is more visual than playable.
They could be deemed as part of the facade in the fortress/green defenses, more formidable looking than they actually are or play.
They do add a unique texture to the golf course, especially on a golf course with so many invisible, deep pit bunkers.
Perhaps they were intended to fool the golfer relying on the obvious, thinking that once past these obstacles, his difficulties were over.
They aren't much of a maintainance problem, and there are some other similar bunkers which are relatively flat that seem related to them.
There are a few pictures of them in "The Garden City Golf Club" "A History"