It can be used in circulation.
As has been pointed out, you can buy fractions of a bitcoin, just like we have fractions of a dollar.
The dollar is limited to 1/100, though. You can chop bitcoin further.
If a vendor accepts bitcoin and you have bitcoin, then you can buy their stuff using bitcoin. Same way you might buy something using PayPal.
The story is similar of cash vs credit.
In the past, few places accepted credit cards for average transactions. You needed cash for more things.
Then as more and more vendors accepted cards and the technology made it easier to administrate business that way, more and more people used cards to pay for things.
If you go overseas, you can buy things in Euros , Swiss francs, etc with the same card you by things in USD.
Your credit card company then translates those charges to USD.
Those cards can adapt to translate bitcoin.
Therefore, the cards could adapt to accept bitcoin to pay your card balance that is listed in USD. You just have to deal with constant currency rate fluctuation.
You could hedge with Bitcoin.
Let's say you think Bitcoin will go up 10% vs USD in one month. Buy 1 Bitcoin at $8,000.
Spend $8,000 on your card. Pay using Bitcoin one month later (or sell for USD). Then have 0.1 Bitcoin or $880 left in your account.
Of course if the value goes down, you lose.
It's gambling the same way you gamble with any currency.