To buy one bitUSD will cost about $1.00 (the actual cost will fluctuate above or below this about, depending on how well the peg is holding).
To short sell one bitUSD, you first have to put up at least $1.75 worth of BTS as collateral. The BTS network then issues you one bitUSD, which you can then sell. You have to maintain your collateral above a certain level (currently 175% of the value of your bitUSD short positions), or you will face a margin call.
You buy bitUSD when you want a currency whose value and volatility will closely follow the dollar.
You short sell bitUSD when you're bullish on the prospects of Bitshares, and you think that BTS will be worth more in the future--relative to the dollar--than it is now.
To illustrate, let's suppose that you borrow one bitUSD from the network, and put up $2 worth of BTS as collateral. Once issued, you sell that bitUSD for $1. Now suppose that BTS rises in price so much that a dollar is now worth only 1% of the BTS it was worth before. You can buy back bitUSD to close out your short position using only $0.01 worth of BTS. In other words, you've bought low ($0.01) and sold high ($1.00), thereby making $0.99 for each bitUSD you shorted.