Think of a bitcoin as a physical bill. I don't know what they are at right now, but when I sold all my BTC each coin was worth $92 each. So someone exchanged their USD for my BTC and they probably used the BTC for a doomsday currency wallet or something. Like most people investing in BTC, they just hoard them for a security net.
Technically, BTC currency is used for purchasing and as payment method online. Most likely, people are wise enough to find different avenues for alternative funds. As much as possible, BTC users try to lessen the use of traditional cash and cards.