Mike, I'll try to summarize, but you'd do better to search for "check chains" posts.
What the chains do is completely eliminate the raising and lowering of the front of the deck caused by the stiff lower 3pt arms. This happens to some degree on relatively flat land and extremely a lot on anything less than flat. If you mow (or brush hog) with the front of your deck supported by the arms (ie not front caster wheels), you would not believe the difference. By using the chains, you now have the deck supported by the huge circumference of the rear tractor wheels. You just would not believe how nice this is until you try it. Also, with this setup, you just drop your 3pt control lever to the lowest position, every time, and you're assured that the deck returns to exactly the same position. Some folks just use the chains, but I wanted more adjustment, so I added the turnbuckles to mine. Some people may use this setup if they have leaky hydraulics, but mine are just fine. My Woods deck came with the setup, but I've modified it just slightly - still use the same brackets at the tractor, but installed eye-bolts at the deck, added snap clips at the tractor end of the chain, and added the turnbuckles. I like this setup so much, that I think if I were to ever buy a new deck that came with front casters, I'd remove them and use the chains instead. I've installed this on two other brush hogs and many other folks on TBN have used this on their equpment.