Seconded. Whilst I don't know the area and thus cannot comment specifically, the most likely reason is either to avoid read-through, i.e. where a different signal could be confused by a driver with this one, and thus a blanking plate is fitted to avoid the second signal being visible, or at least to reduce it's visibility, or to reduce other sources of light, usually the sun but not always, that can either make it harder to see the signal or possibly give phantom aspects. Phantom aspects are much less of a problem with LEDs though, so probably more just to help the drivers actually see it clearly when the sun is behind the signal.