All you gotta do is go to Google Maps, type in a route you want to see, and then look for the 3D Play button that should pop up next to "Driving Direction to _______". You can follow your little dot as it speeds along your route, whipping past 3D buildings (if they are available in your area).
The smoothness of your experience is somewhat dependent on your internet connection speed and your computer's processing power. There's basically no purpose to it other than geeky-coolness, and geeky-coolness for geeky-coolness' sake, is a-okay with me. Weeee! [Google Maps Blog]