CoinDesk Bitcoin Price Index

Info on how this value was calculated
Based on the following values:
USD:
BTC-e$486.00
Bitstamp$497.39
Bitfinex$497.76
CNY:
BTC China¥3,094.20
OKCoin¥3,092.95
Last updated: Apr 19, 2014 at 18:01 BST

Why use bitcoin?