Exposé shows all the windows of the app. When you use it with only one window open, it's pretty pointless, as you've seen. But when you have more than one window open for a certain app, it's really useful. Trying opening up a few Safari windows and then using it.
oh i get it now ive just tried it with 2 chrome windows, seems a bit pointless to limit it to one app. i had more than one open in my screenshot above, seems more useful to show all apps like it used to as thats a far more likely scenario to be in
Exposé no longer exists since Lion. It has been replaced by Mission Control, which tries to do something like Exposé, Spaces and Dashboard at the same time and fails spectacularly at it. Mission Control might as well grate cheese and download every Spice Girls album while it's at it, it would make as much sense.