Yeah, pretty much for extra distance on a cast. Their major downfall is that they won't show "uplift" bites, where the fish comes up and grabs the "bait", which takes the downward pull of the weight of that bait away from a non-weighted float, causing it to fall over (or at least rise up some or lean).
Yes ... putting a weight on the float or directly below (or even well below) the float would accomplish the same thing ... but would also suffer the same downfall.
Float n Fly anglers use a float that has a weight stuck into the "side" of the float. It's a counterweight to the jig hanging below it. When a fish comes up and takes the jig into their mouth & doesn't immediately dive down with it .... the float will roll over on the weighted side, indicating a lift bite. They are not simply the foam floats with a lead ring around the bottom, but are foam floats with the counterweight inside the float itself.