913 Driver and 3 Wood

What benifit if there in changing the weight in my 913 3 Wood or Driver?

It gives you the ability to change the way the club feels when you swing it.  For instance, If you have a fast tempo a heavier club will slow down your swing and give you potentially a smoother rhythm.  In my case, that is why I use heavier clubs being a bigger guy (I would like to say stronger but I am just feeding my ego at that point! LOL).