It could also be a linguistic barrier. If the singer is from the same country, they could get away with shouting “all girls scream”, but they still risk not being understood by parts of the audience… When I’ve experienced it it was always visual, either it would be two different singers in charge of their own “area” of the public, or the singer would “move” right/left and “mimic” the specific part of the audience to scream (sorry English is not my native language so I’m not sure how to convey this…)