As an avid float angler, bulk shot or weights are used to get the bait down quickly to the desired depth, and for stability in rougher water. Spread out shot allows for a slower drop of the bait through the water. Also, when holding back with spread out shot, the bait can rise over obstacles in the water. Once past the obstacle, let the float go at the speed of the current and the bait will drop again.
Mind you, of all the anglers I've seen on the Vedder, most are using large shot such as AAAs spread 9-12ins apart. It looks ungainly and I'm not quite sure whether it will give the right effect. Much better to use more smaller shot on the line spread "shirt-button" style. This works really well on calmer flowing water. It is a style used extensively in the UK to seach through the water column for fish which rise up to the bait (due to constant loose feeding). If the fish are holding on the bottom, then I would suggest a bulk weight about 15-18ins off the bottom and maybe one dropper is more effective.