"Why would you want Bitcoins instead of cash?". For a second I didn't know what to say; then I thought about it and here is what I came up with. Feel free to contribute to this.
1) More security (especially if you are careful).
2) Bitcoins are a potential investment, cash is not. Paper currency only really goes down in value, Bitcoins have a lot of potential to be worth more as time goes on.
3) Fast, worldwide transactions at your fingertips.
4) Similar to #3, but it's a universal currency. It's legal in most countries of the world and you don't need any kind of currency conversion to send money to people in a different country than your own.
Some points I want to make clear why BTC is better than actual cash
1.) I can actually save my BTC (money)
2.) Cash will stay in the same amount from day 1 to the days you are going to spend it. BTC goes up and down depends on the rate. My BTC can value to higher money if I get lucky.
3.) BTC is actually SAFER