There are alot of reasons to do this. As Alan mentioned, readiness is alot of it. Not just to defelct rejected passes, but to know what position the rod is in, thus where the men are located. Knowing where every one of your men is at all times is VERY important. That way you know what holes are being covered at all times. But in rereading your post, do you mean when they are playing DEFENSIVE 5-rod? That is also very similar reasoning to what I previously stated. This could turn into a very long post. I will try to write it down and post a more in depth response soon.