Line stops and beads are used to stop movement on the line.
Line stops can be used with wagglers to trap them in place. They can also be placed either side of a hooklength swivel on a helicopter rig, or used on a free running rig to stop the feeder sliding down.
Beads and sleeves are used alongside line stops to protect knots and improve presentation. Used correctly, line stops help keep rigs exactly where you want them when float or feeder fishing.