Practical: Over-radiator fans - are they really a good thing?
Central heating rads are either against a plain wall or often situated just under a window
Wall : without a fan the rad's heat will be divided between convection into the room and heating the wall. Maybe a fan here WOULD warm the air more quicjkly but heating the wall is not a bad thing as it too can radiate and in a stable environment the wall WILL adopt room temperature
WINDOW: Windows are a major source of heat loss - even double-glazed windows will feel very cool to the touch when it's cold outside. Remember heat rises! So the warm air from the rad rises to meet the cooler air dropping from the window and THAT is why so many rads ARE situated under windows. Not sure a fan will really do much for that balance!
Heavy curtains or blinds can hold back the cooler air somewhat BUT importantly do not hang curtains so long that they also cover the rad otherwise the heat is only warming the window area and doing little or nothing for the room!