Cash is dirty and full of bacteria, bitcoin is digital clean and doesn't rot.
I like this reason the most!
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.
5) Banks can't steal it from you. You may think they can't steal cash either; that's not true. With central banks printing money by the hundreds of billions, and commercial banks creating money as debt like there's no tomorrow, they're effectively lowering the relative worth of your money in their advantage. Remember: stealing 5% off your deposit, or increasing the total money supply by 5% by creating it out of thin air, has essentially the same effect.
6) Bitcoin can be in virtual and physical state simultaneously without the need for conversion I.e. online wallets and, for example,
PaperSafe 7) Bitcoins can be backed up. When you lose a wallet with cash, it's gone. When you lose a wallet with bitcoins, you still have a backup.
8) Bitcoins can be password protected. When someone steals your wallet with cash, it's gone. When someone steals your wallet with bitcoins, they can see how much is in there, but they can't spend a dime.
9) It is far easier to carry 500 BTC with you on a plane than 500 BTC worth of cash.
And, of course these bulletined items make up the main benefits of Bitcoin over cash. As someone who has lost a wallet more than one time (with cash) as well as had an apartment broken into and all the cash stolen I can appreciate the password protected and back up feature of Bitcoin feature. But also for storing, and carrying larger amounts of money it is far easier like mentioned above to carry 500 BTC on a plane than $146,225 at today's rate of 1 BTC = $292.45.