Usually choirs are in the back above in choir loft. The head pastor is to blame to allow this. Yes, huge distraction.
So, I imagine that it's something that leaked from the seminary. In a seminary setting, the choir effecitvely consists of all the clerics (those who had received tonsure, could wear the cassock, etc.), i.e. most of the seminarians, and the choir is situated up front near the sanctuary. In that case, any schola would organize in the front of the church, just because that's where they were anyway. Choir lofts were a thing for churches where you didn't have lots of clerics up front (i.e. most parish churches) and you had lay choirs. If there was a visiting cleric, they'd often sit in the sanctuary during Mass / Office. Nevertheless, even with the seminary scholas, I never saw the need for a "flapper". IMO they serve no purpose as everyone's looking at their music sheets or
Liber Usualis and barely noticing the flapper, just like with most orchestras also. Conductors train all the musicians beforehand but then during the performance they're unnecessary other than to stand up there to take credit for the performance and so the audience has something to look at. Also, many ancient churches that had clerical choirs would have some fairly high stairs going up to the altar, so that the choir wouldn't necessarily obstruct your view of the sanctuary too much.