Usually the planks under a balcony are protected by the balcony itself, so they remain fairly dry, especially if there are drip flashings that redirect rain drips and rivulets away. Or, they may be pressure treated wood, in which case it's likely to outlast the building whether it gets wet or not. There might be a fascia board that is designed to take the rain and rot away, separated from the structural beams by a flashing, vapor barrier, standoffs, hidden paint/stain, or nothing!