The beautiful, billowy bed curtains we associate with elegance and style, actually have a rather ordinary and functional history. In those drafty old European castles of old, the lord and his family slept in the great hall along with all their servants, because this hall was one of the few areas of the castle where the fires were kept stoked for warmth and light. The noble family's sleeping area was separated from the rest of the riffraff by curtains. Originally, the curtains were hung from the ceiling, but as the bed evolved, a frame was added to support a canopy from which the curtains hung.
Well ... there's nobility involved in that story, which qualifies as 'princessy', which makes them perfect for our teen room makeover. They also add a strong vertical element to counteract the many horizontal surfaces in the room.