Yeah we need both of them at a moment as we cannot afford to depend only on bitcoin as bitcoin is not accepted everywhere at a moment and we need fiat to take care of our purchases.
but at some stages we can rely only on bitcoin, like if we buy only those goods which are available online then we will buy them with bitcoin (those bitcoins which we earn online from our services/work) , with that we do not need to deposit or withdraw them via bank. and if a time came when everything will be available with bitcoin then we all will leave banks for our support to bitcoin.