Basically If I buy something for a dollar, I know that it will still be worth about a dollar in the future, Maybe less due to inflation, But with bitcoin Because there is a limited amount that will ever exist, It's considered more of a commodity rather than a currency, so there's a higher possibility that the price could go up due to higher demand. So that one dollar of bitcoin could eventually be worth 10x more than it is now.
It's like the guy who spent 10,000 bitcoins on a pizza, back when the value of bitcoin was extremely low, Because the value has now risen by such a large amount, that 10,000 bitcoin would now be worth almost $6million. For one pizza!
Look at it this way, If you had some gold stashed away in your safe, and you also and fiat currency in your wallet, Which one would you take with you to the shop to buy milk? I'm guessing that you said fiat, Otherwise, I'd like to know where you shop XD

Imagine being this poor guy. http://www.forbes.com/sites/ericmack/2013/12/23/the-bitcoin-pizza-purchase-thats-worth-7-million-today/#4b68c6f96449