Imagine you are a city or any other public administration with urban planning powers. How to foster the use of a given architectural shape without paying for it? Reducing the cost for those producing it. In a given moment, the city of Madrid decides that the floor area of a bow window is accounted for just about 50% of its size in the overall floor area permitted by the municipal plan on each lot. So it is a more profitable space than other square meters in the building. This explains why you so often this shape in the Madrid architecture of the last two decades.
Is this a better architectural solution? A more elegant one? You cannot say, as this depends on each project. Conversely, some cities as Barcelona are much less welcoming towards these bow windows, and this has been a tradition for more than a century. It is a matter of local sensibility… Barcelona’s position derives from the overcrowding in the old city before the Cerda extension in mid XIXth century, when cantilevering rooms sometimes covered the street. I could not trace back the reason for Madrid’s permissiveness.