Bitcoin is allowing "just-in-time" cash management of a bank account.
This lets the person who gets bitcoin income to use a bank account only when needed (e.g., mortgage or rent payment, etc.) A transfer from Coinbase takes two to three business days, so if you pay monthly, for instance, you can wait right up until the end of the month to sell your coins and have the cash arrive at about the same time the payment is due.
One might think this is risky because there could be issues -- such as Coinbase canceling the order as "high risk" (which has happened to many, reportedly) but then there is also the same-day backup plan, using
http://Bitcoin-brokers.org (or similar) to get a trade where cash is deposited into your bank account.
So while a person still might keep a bank account, Bitcoin enables one to manage a lower bank account balance.