it gets rid of the stock unit bearings, and seals that are $$$. it replaces them with much more common bearings and seals. i screwed up some how and damaged a inner hub seal and it was 12.00 for a new one. you also get stronger outshafts, and a fixed spindle.
the only down side is if your truck has the shift on the fly, you loose the auto hubs option. but you get a stronger hubs.
if your unit bearings do go out, get ready to spend some $$. from what i have heard usually the bearings go out without any warning and cause alot of damage.