When the good weather starts, one must assume an objective fact: Barcelona, in the summer, is a hot city.
And long before there was air conditioning, it was already hot. So the locals had to protect themselves against the sun. And they found a solution that is ideal from my point of view: the shutters on the outside, leaving them hanging over the balcony.
Thus they stop direct sunlight, but they also let the air in in order to get the natural cross ventilation (if there is a little breeze blowing, the heat is more bearable) and, unlike horizontal blinds, they preserve a certain degree of privacy.
I will not say I thank an air conditioner in the summer in the city, but if we don't have one... what could be better than the these typical blinds?