1) Superior bite detection
2) Increased casting distance
3) Ability to rig as fixed or slip
YMMV
True, years ago I would use a porcupine quill, and sandwich the medal loop between two spit shots to lock it in place. And only enough shot for just the tip to show...very sensitive.
But, with a slip float, you cast into nooks and cranny that a pegged float can't. Also if you slowly crank reel handle back to you, it will achieve the same pendulum action as a pegged float .