Chris, it works on both pole line/floats and running water as well. Rob is making reference to running water floats i think.
But i use it on both disciplines and on windy days i pays dividends. I do not remove any shot below the float.

On running water, with a stick float, a back shot helps to slow the speed at which you run the float through, you simply hold a tight line to the backshot and ease the float through.
If there is a skim on the river this helps to keep the float on line too. If you dont hold onto the backshot your float will sink.
On still water pole fishing, back shot are more likely to be held out of the water to help to straighten out the line to the float. For F1 carp for example the idea is it reduces missed bites by allowing you to hold a tight line to the float, a bit of a bolt rig if you like.
If its blowing hard you can sink the back shot on the pole or waggler to combat drift but you need to adjust the shotting below the float otherwise the extra back shot would sink it.
Hope the above makes sense.
If you read the early books on river fishing by the likes of Ivan Marks or Clive Smith they were using BBs for backshot, on modern pole rigs its more likely to be 8s or 10s.

Ive had a play several times at backshotting and it was only last week pleasure fishing in awfull conditions that i made it work very well,i had 24 inch of line between my float and pole tip and i strung 4 no 8 about 10 inch from the float this gave me some thing to hold tightly to rather than the float and stabilised the rig I will definetly do it again. I think ive gone wrong before trying to backshot using to shorter line, my mate does it and he bulds one of his back shots into his rig that way if the lights poor etc you can slide the back shot towards the pole tip and lift your float.