These rubber buffer beads are used to cushion impacts across the rig when casting or playing a fish. These beads can sit on the mainline above the float to provide a cushion between the float and the stop knot or float stop.
Equally, they can be used below the float to absorb the shock of the ball weight running up the line and banging into the float. Also, between the weight and the trace to cushion the trace swivel.
They provide excellent knot protection when using running rigs, helping to protect the knot when casting heavier weights and baits to distance.