the wing nut holds the stiff spring from pushing the grease out of the seal prematurly if it gets too warm or the seal fails.... the wing nut can be backed off an eighth or quarter of a turn just before backing the trailer in the water to let the spring pressure down on the grease...
the object and purpose of all types of bearing buddys are to maintain pressure on the hub so that when you back into the cold water, the instant cooling effect doesnt suck cold water in thru the seal.... the spring will have a preload to fill the void if it needs to, rather than the water filling it.
the wing nut version is just someones idea of a cleaner way of doing things and so the seal doesnt have constant pressure on it at all times. I think the spring may be a bit stiffer than the other types but it doesnt seem to be any better or worse, its just different....