From the Manual on Uniform Traffic Control Devices (as quoted in Wikipedia)
A. Assist cyclists with lateral positioning in a shared lane with on-street parallel parking in order to reduce the chance of a bicyclist impacting the
open door of a parked vehicle;
B. Assist cyclists with lateral positioning in lanes that are too narrow for a motor vehicle and a bicycle to travel side by side within the same traffic lane;
C. Alert motorists of the lateral location bicyclists are likely to occupy within the traveled way;
D. Encourage safe passing of bicyclists by motorists; and
E. Reduce the incidence of wrong-way bicycling.
So the ones I see are in effect telling everyone "The road isn't wide enough to try to pass a bike here" (point B). The others seem less relevant.