The built-in speaker in the iPhone 4 is fine, but it's not powerful enough to really share music with a group of people. It'll work in a pinch, but you're not going to want to use it for any real amount of time.
But the iPhone Horn Stand aims to make the speaker a bit more powerful without adding to the drain on the phone's battery. It uses its shape to amplify the sound without any power necessary. It can be used to stand the iPhone up either horizontally or vertically, and at $25 it's cheaper than most docks that promise to do the same thing.