Everywhere in Australia, I have seen buildings with balconies that overlook busy roads. The view from the balcony often only includes other buildings. These balconies seem useless, because not many people want to sit in the street noise and car exhaust. I have rarely seen anyone on these balconies, and then only moving around for a practical purpose, not enjoying the air and view. Mostly the balconies are used for storing unwanted furniture and sports equipment, or growing potted plants. This makes sense, because even drying laundry over a busy road is problematic – everything gets covered in fine black soot. What does not make sense is adding these balconies to the buildings in the first place. A more practical use of the space would be to close the open parts of the balcony and thus add an extra room to the apartment. It is used as a storage room anyway.
In some cases, the building might have been constructed before the street became too busy or the views blocked by other buildings, but most of the buildings with balconies are new, so this explanation does not apply.
The reason the developers add balconies to their buildings is probably to market the apartments to impractical people. An included balcony makes the apartment sound more luxurious, and usually the view and relaxation opportunities of the balcony are touted in the advertisement. But people inspect the apartment before buying, so they should see the uselessness of the balcony for anything but storage. Inspections are usually scheduled on Saturdays when there is less traffic, and the inspecting buyers don’t sit on the balcony for long enough to become annoyed by the noise and the car exhaust.
There may be rules against the owner of an apartment closing up the balcony to create a room, because this makes the building facade uneven. Coordination problems between apartment owners may prevent them from closing up all the balconies of the building simultaneously.