To quote the “Advanced Mac OS X Programming” book:
“This is useful for programs that need to manage the order in which file descriptors are allocated.”

I’m not sure how helpful this is.

In a later part of the book they go on to describe how “when you open a file, the file descriptor returned is guaranteed to be the lowest numbered unused descriptor.” And by default stdin=0, stdout=1, and stderr=2. Perhaps there were applications that were making assumptions about file descriptor numbers…