It's to keep the hook snood "proud" (standing away from the rest of the trace) and also to counter the line twist (of the hook snood) that invariably happens when retrieving un-/half munched bait.
You'll find with a normal two-eyed swivel that the hook snood twists up and wraps around the trace after retrieving. Also the bait hangs away from the sinker snood and presents better than with a two-eyed swivel.
Having said all that I still just use two-eyed swivels. Never real found it to be such a big deal.