they do in theory increase sustain - anything thats stops helps to stop unwanted vibration will do that, but we dont really need much more sustain anyway!!
I think they just make the bridge transfer string energy slightly more effeciently due to it all being locked togther as one unit, but it should be held quite firmly under string tension anyway
The best bit is that your bridge doesnt fall off or move slightly when you change all the strings or abuse your guitar!
I like them but i think they are a bit overpriced for the small benefits they give, so not really necisarry - but why not have them if you can afford them