Hardly a security measure. If an 'extra' non-BP holder gets on, the headcount will be wrong. Or don't you do those either ?
If the interloper is a terrorist, then checking the BP at entry won't help. He/she is already on the plane.
It is useful only to ensure the pax is on the right flight. Of most use when boarding is done from a ramp full of airplanes.
Wouldn't surprise me if it is a government reg. in places. Just the sort of feel-good, easily implemented but largely ineffective procedure they seem to concentrate on.