Communications is at the heart of diplomacy... diplomats both speak and listen on behalf of their governments. It's not surprising, then, to find modern telecommunications antennas gracing the tops of century-old mansions.
The original premise of ambassadors and embassies was that governments needed representatives in foreign capitals, to speak for governments separated by weeks of travel. This situation has clearly changed, as world leaders can be a phone call away, and CNN and other media services make news of world events accessible to everyone. Some diplomatic functions have changed as a result, but many remain the same, or are complemented by new technologies (so the ambassador is still needed for that face-to-face negotiation, but carries a cell phone in her briefcase, and holds weekly videoconferences with officials at the foreign ministry).
Protesters can also take advantage of Washington D.C.'s nature as a diplomatic hub to communicate to governments, as these Greenpeace activists did in August, 2000, inflating an immense blue whale balloon in Dupont Circle to draw attention to continued Japanese hunting of whales.