What is the usefulness of stations CHU, WWV and WWVH to amateur radio operators?

CHU, WWV, and WWVH are government-operated shortwave radio stations that broadcast highly accurate time and frequency signals continuously. CHU is operated by Canada, while WWV and WWVH are operated by the United States. Amateur radio operators use these stations for two primary reasons: to synchronize their station clocks precisely (which is especially critical for digital modes) and to calibrate their radio receivers to ensure their frequency displays are accurate. While listening to these stations on various frequencies can informally give you an idea of current propagation (since you can see which bands are "open" to the station's location), their engineered purpose is strictly to provide standard time and frequency markers.