Because when they are on standby, they can be sent anywhere in the US or Canada to operate a flight, e.g. YVR, YYZ, JFK, ORD, or LAX. They will also operate regional flights out of HKG. I just had a look at the 777 pilots based in North America, they all have at least one regional flight out of HKG (e.g. PER, PVG, MNL. SIN) on their roster.