The term standard siren has been coined for the use of GW detections in the manner of a standard candle, e.g., to determine the Hubble constant. The word "siren" is used rather than "candle" to indicate that it is not based upon the brightness of light (EMR), but from characteristics of the GW transient's chirp. Though obviously of use in building the cosmic distance ladder, it is an independent measurement, i.e., it is a long distance measurement that is not dependent upon the ladder of other shorter distance measurements.
Though the EMR brightness is not key to the use of a standard siren, observed characteristics can be used to refine the measurement, making GW events that produce EMR (e.g., neutron star mergers) the most useful: its indication of a more precise direction of the event reduces the distance error bars, and models of the type of event (e.g., kilonovae) can relate observable characteristics to specific orbital inclinations, also a factor in the distance determination from the chirp.
As the number of such detections grows, error bars on the determination of the Hubble constant will shrink.