You can put plant vases on those and not have the risk of your vases falling on someone's head. Somewhat like this: http://www.apartmenttherapy.com/look-flower-pots-as-window-b... (random image found with google image search).

My theory is that they are there to prevent people from either A) plummeting to their death upon climbing out the window or B) being able to throw trash out their windows into the street

There's no way they are useable as a balcony as you'd have to climb out a tiny window and drop 4 feet onto a sharp spiked railing.

Maybe to support air conditioner or just for plants (decoration, just to see some green through the window).

I'm guessing it's just superficial decoration, as there is no door to make it useful as balcony.