To exchange it, someone has to want to BUY Bitcoins on the other side. Who does that today? Mostly speculators hoping to cash in on the rise of Bitcoin value.
There are benefits to turning your USD into bitcoins. One reason is you control your money. No one can pump money into the system to devalue your dollars. Pseudo-anonymity being another reason. The fact that bitcoin is bankless (and therefore fee-less) is another reason. The fact that I can send coins to anyone around the world without converting and reconverting between different currencies is another reason. The fact that any number of coins can all fit into a postage stamp sized SD card is yet another benefit.