So they can just fail you flat out for no reason if they wanted to?
/disclaimer
My experiences were in the dark ages of the '70s
/end disclaimer
When I went through, "They" could sense those who were on the edge of failure, and hasten the process. If the candidate "got it all in one sock," and started improving, that was good. If the candidate continued his downward spiral, then he probably wasn't suited for the military, and more importantly the aviation military. They stressed "attention to detail." Later on, in flight training, one came to the realization that there was a purpose to their methods.
I can't speak for today's OCS, but i imagine that things aren't that much different overall.