Pegging. There has been much discussion on the topic (see hang back rigging) in a number of different contexts though conceptually the principles are the same.

Separating your bait or lure and the hook greatly reduce the chances of the fish taking a hook deep. Using the hangback method, the fish takes your offering and the hook gets them, more often than not, on the outside of the mouth, or rather from the outside in. In other words, it's better for the fish and I think that is the most important of the reasons. Some of the rest are debatable.

I do actually think that my hooked to landed ratio is just as good if not better using this rigging as opposed to having the yarnie or similar small offering sitting directly on the hook. Small as something like a yarnball may be, I still think that even a yarnie buffers the hooks ability gain maximum purchase by either blocking some of the gap, or creating some turbulence just as the hook is trying to seal the deal. With some space between lure and hook, there is nothing to prevent the hook from doing it's job as best it can.

Another reason. I like my yarnies, corkies, beads, whatever to be free sliding on the leader. I have it in my head that this can lead to some slightly more "natural" action in the water with the lure having a little more room to move as compared to it being fixed in place. Whether or not there is any truth to this idea or not I can't say for sure, but if it ain't broke...

I am sure others (Doc!) can illustrate the finer points of hanging it back much more eloquently than I can.