At a guess it's to avoid a conflict with Yankee over a noisy channel. (I.e. if you hear "[static] ank [static]" did you hear "Frank" or "Yankee"? "Foxtrot" prev

At a guess it's to avoid a conflict with Yankee over a noisy channel. (I.e. if you hear "[static] ank [static]" did you hear "Frank" or "Yankee"? "Foxtrot" prevents that conflict.)