I can exchange them fast and pay cash for my things
Yes.In place where bitcoin shops are not available,you can always convert them to your local currency or to USD and then USD to your local currency.This has opened many possibility for bitcoin users
But this adds lots of fees and every time you convert one currency to another you lose some money too and then you add banks fees as well, one of the purposes of bitcoins was to allow users to bypass fees.
The moment you convert your bitcoins the advantage of low fees is gone. And you are indeed better off by just buying it in fiat. Using a card helps but for me using bitcoin is mostly online, not as much as I wish I could but still i use it.